shot peening residual stress - Metal Improvement Company

T A B L E O F C O N T E N T S 

T A B L E O F C O N T E N T S 

Conversion Tables................................................................................. 2,3 

Introduction ............................................................................................. 5 

CHAPTER 1 : 

THEORY 

The Shot Peening Process........................................................................ 6 

Shot Peening Residual Stress .................................................................. 7 

Summation of Applied and Residual Stress ............................................. 8 

Application Case Study: NASA Langley Crack Growth Study .................... 8 

Depth of Residual Stress.......................................................................... 9 

Shot Peening Media ................................................................................. 9 

Effect of Shot Hardness............................................................................ 9 

CHAPTER 2 : 

RESPONSE OF METALS 

High Strength Steels .............................................................................. 10 

Carburized Steels.................................................................................... 11 

Application Case Study: High Performance Crankshafts ......................... 11 

Decarburization ...................................................................................... 11 

Application Case Study: Reduction of Retained Austenite...................... 12 

Austempered Ductile Iron....................................................................... 12 

Cast Iron................................................................................................. 12 

Aluminum Alloys..................................................................................... 13 

Application Case Study: High Strength Aluminum.................................. 13 

Titanium ................................................................................................. 14 

Magnesium............................................................................................. 14 

Powder Metallurgy ................................................................................. 15 

Application Case Study: High Density Powder Metal Gears.................... 15 

CHAPTE R 3 : 

MANUFACTURING PROCESSES 

Effect on Fatigue Life .............................................................................. 16 

Welding .................................................................................................. 16 

Application Case Study: Fatigue of Offshore Steel Structures ................ 17 

Application Case Study: Turbine Engine HP Compressor Rotors ............ 17 

Grinding ................................................................................................. 18 

Plating .................................................................................................... 18 

Anodizing ............................................................................................... 19 

Application Case Study: Anodized Aluminum Rings............................... 19 

Plasma Spray.......................................................................................... 19 

Electro-Discharge Machining (EDM) ....................................................... 19 

Electro-Chemical Machining (ECM) ........................................................ 20 

Application Case Study: Diaphragm Couplings...................................... 20 

CHAPTER 4 : 

BENDING FAT IGUE 

Bending Fatigue ..................................................................................... 21 

Gears...................................................................................................... 21 

Connecting Rods.................................................................................... 22 

Crankshafts............................................................................................ 23 

Application Case Study: Diesel Engine Crankshafts............................... 23 

Application Case Study: Turbine Engine Disks....................................... 23 

CHAPTER 5 : 

TORSIONAL FATIGUE 

Torsional Fatigue.................................................................................... 24 

Compression Springs............................................................................. 24 

Drive Shafts ........................................................................................... 25 

Torsion Bars........................................................................................... 25 

Application Case Study: Automotive Torsion Bars ................................. 25 

CHAPTER 6 : 

AXIAL FATIGUE 

Axial Fatigue .......................................................................................... 26 

Application Case Study: Train Emergency Brake Pin.............................. 26 

Application Case Study: Auxiliary Power Unit (APU) Exhaust Ducts....... 26 

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CHAPTER 7 : 

CONTACT FAILURE 

Fretting Failure....................................................................................... 27 

Application Case Study: Turbomachinery Blades and Buckets............... 27 

Pitting .....................................................................................................27 

Galling ................................................................................................... 28 

CHAPTER 8 : 

CORROSION FAILURE 

Corrosion Failure.................................................................................... 29 

Stress Corrosion Cracking...................................................................... 29 

Application Case Study: Fabrication of Chemical Handling Equipment .... 30 

Corrosion Fatigue................................................................................... 30 

Application Case Study: Sulfide Stress Cracking.................................... 30 

Application Case Study: Medical Implants ............................................. 31 

Intergranular Corrosion .......................................................................... 31 

CHAPTER 9 : 

THERMAL FATIGUE & HEAT EFFECTS 

Effects of Heat ....................................................................................... 32 

Thermal Fatigue ..................................................................................... 33 

Application Case Study: Feedwater Heaters........................................... 33 

CHAPTE R 10: 

OTHER APPLICATIONS 

Peen Forming ......................................................................................... 34 

Contour Correction................................................................................. 35 

Work Hardening ..................................................................................... 35 

Peentex sm ............................................................................................... 36 

Engineered Surfaces .............................................................................. 36 

Application Case Study: Pneumatic Conveyor Tubing............................ 37 

Application Case Study: Food Industry .................................................. 37 

Exfoliation Corrosion.............................................................................. 38 

Porosity Sealing..................................................................................... 38 

CHAPTER 11: 

ADDITIONAL C APABILITIES & SERVICES 

Internal Surfaces and Bores................................................................... 39 

Dual (Intensity) Peening ........................................................................ 39 

The C.A.S.E. sm Process ............................................................................ 40 

On-Site Shot Peening ............................................................................. 41 

Strain Peening ........................................................................................ 41 

Peenstress sm Residual Stress Modeling .................................................. 42 

Lasershot sm Peening ............................................................................... 43 

Valve Reeds – Manufacturing................................................................. 43 

Reprints/Technical Articles .................................................................... 44 

Heat Treating ......................................................................................... 44 

CHAPTER 12: 

CONTROLLING THE PROCESS 

Controlling the Process.......................................................................... 45 

Media Control ........................................................................................ 45 

Intensity Control .................................................................................... 46 

Coverage Control ................................................................................... 47 

Automated Shot Peening Equipment ..................................................... 48 

Application Case Study: CMSP Increases Turbine Engine Service Life ...... 49 

Specifying Shot Peening ........................................................................ 50 

MIC Technical Reprints ..................................................................... 51-55 

MIC Facility Listings ................................................................ Back Cover 

1

C O N V E R S I O N S 

2 

Approximate Conversion from Hardness to Tensile Strength of Steels 

Rockwell Brinell Vickers Tensile Tensile 

Hardness Hardness Hardness Strength Strength 

HRC BHN HV ksi MPa 

62 688 746 361 2489 

61 668 720 349 2406 

60 649 697 337 2324 

59 631 674 326 2248 

58 613 653 315 2172 

57 595 633 305 2103 

56 577 613 295 2034 

55 559 595 286 1972 

54 542 577 277 1910 

53 525 560 269 1855 

52 509 544 261 1800 

51 494 528 253 1744 

50 480 513 245 1689 

49 467 498 238 1641 

48 455 484 231 1593 

47 444 471 224 1544 

46 433 458 217 1496 

45 422 446 211 1455 

44 411 434 206 1420 

43 401 423 201 1386 

42 391 412 196 1351 

41 381 402 191 1317 

40 371 392 186 1282 

39 361 382 181 1248 

38 352 372 176 1214 

37 343 363 171 1179 

36 334 354 166 1145 

35 325 345 162 1117 

34 317 336 158 1089 

33 309 327 154 1062 

32 301 318 150 1034 

31 293 310 146 1007 

30 286 302 142 979 

29 279 294 138 952 

28 272 286 134 924 

27 265 279 130 896 

26 259 272 127 876 

25 253 266 124 855 

24 247 260 121 834 

23 241 254 118 814 

22 235 248 116 800 

21 229 243 113 779 

20 223 238 111 765 

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Common Conversions Associated with Shot Peening 

Metric (SI) to English (US Customary) English to Metric 

Length 1 mm = 0.0394 in 1 in = 25.4 mm 

1 m = 3.281 ft = 39.37 in 1 ft = 0.3048 m = 304.8 mm 

Area 1 mm 2 = 1.550 x 10 –3 in 2 1 in 2 = 645.2 mm 2 

1 m 2 = 10.76 ft 2 1 ft 2 = 92.90 x 10 –3 m 2 

Mass 1 kg = 2.205 lbm 1 lbm = 0.454 kg 

Force 1 kN = 224.8 lbf 1 lbf = 4.448 N 

Stress 1 MPa = 0.145 ksi = 145 lbf/in 2 1 ksi = 6.895 MPa 

Miscellaneous Terms & Constants 

lbm = lb (mass) lbf = lb (force) 

k = kilo = 10 3 M = mega = 10 6 

G = giga = 10 9 µ = micro = 10 –6 

1 Pa = 1 N/m 2 lbf/in 2 = psi 

ksi = 1000 psi µm = micron = 1/1000 mm 

Young’s Modulus (E) for Steel = 29 x 106 lbf/in2 = 200 GPa 

Acceleration of Gravity = 32.17 ft/s2 = 9.81 m/s2 Density of Steel = 0.283 lbm/in 3 = 7.832 x 10 –6 kg/mm 3 


C O N V E R S I O N S 

3

N O T E S 

4 


Metal Improvement Company, Inc. (MIC) is a wholly owned subsidiary of the Curtiss-Wright Corporation. 

Founded in 1946, MIC specializes in providing shot peening services to many industries as a means of 

preventing metal failures. MIC operates shot peening plants throughout North America and Western 

Europe with process licensees worldwide. Additionally, MIC operates a network of heat treating 

facilities and manufactures reed valves. A complete facility listing is printed on the back cover of this 

publication. 

Each MIC shot peening plant is capable of processing parts of diverse shapes, sizes and materials under 

rigidly controlled conditions. MIC develops the latest, state-of-the-art peening equipment based on the 

body of experience gathered over more than 50 years in the business. 

Metal Improvement Company, Inc. is the recognized leader in developing applications and controls used 

in shot peening. Shot Peening Applications – Eighth Edition replaces the Seventh Edition as the 

foremost engineering manual on the controlled shot peening process. 

MIC has available "Shot Peening Applications - The Video" which, especially when part of a live 

presentation by one of our Technical Service Managers, is the ideal medium for a company group or a 

meeting of a technical society. For more information you may contact the nearest MIC shot peening 

facility (see back cover), our World Service Headquarters or visit our web site at 

www.metalimprovement.com. The Eighth Edition is also available in French, German, Spanish 

and Italian. 

MIC is pleased to share its experience and knowledge in the shot peening field so that engineers and 

metallurgists may be aware of the benefits of shot peening. Our Technical Service Managers are 

available to assist in the solution of many engineering problems that can be overcome by shot peening. 

Copyright © 2001 

By 

Metal Improvement Company, Inc. 


I N T R O D U C T I O N 

5

C H A P T E R O N E 

THEORY 

6 

T H E S H O T P E E N I N G 

P R O C E S S 

Shot peening is a cold working process in which the 

surface of a part is bombarded with small spherical 

media called shot. Each piece of shot striking the 

metal acts as a tiny peening hammer imparting a 

small indentation or dimple on the surface. In order 

for the dimple to be created, the surface layer of the 

metal must yield in tension (Figure 1-1). Below the 

surface, the compressed grains try to restore the 

surface to its original shape producing a hemisphere 

of cold-worked metal highly stressed in compression 

(Figure 1-2). Overlapping dimples develop a 

uniform layer of residual compressive stress. 

It is well known that cracks will not initiate nor 

propagate in a compressively stressed zone. Since 

nearly all fatigue and stress corrosion failures 

originate at or near the surface of a part, 

compressive stresses induced by shot peening 

provide significant increases in part life. The 

magnitude of residual compressive stress produced 

by shot peening is at least as great as half the 

tensile strength of the material being peened. 

In most modes of long term failure the common denominator is tensile stress. These stresses can result 

from externally applied loads or be residual stresses 

from manufacturing processes such as welding, 

grinding or machining. Tensile stresses attempt to 

stretch or pull the surface apart and may eventually 

lead to crack initiation (Figure 1-3). Compressive 

stress squeezes the surface grain boundaries 

together and will significantly delay the initiation of 

fatigue cracking. Since crack growth is slowed 

Figure 1-3 Crack Initiation and Growth 

significantly in a compressive layer, increasing the 

Through Tensile Stress 

depth of this layer increases crack 

resistance. Shot peening is the most economical and practical method of ensuring surface 

residual compressive stresses. 


Figure 1-1 Mechanical Yielding at Point of Impact 

Figure 1-2 Compression Resists Fatigue Cracking

Shot peening is primarily used to combat metal fatigue. The following points pertain to metal fatigue and 

its application to the Typical Stress versus Load Cycles graph shown in Figure 1-4. 

• Fatigue loading 

consists of tens of 

thousands to millions 

of repetitive load 

cycles. The loads 

create applied tensile 

stress that attempt to 

stretch/pull the 

surface of the 

material apart. 

• A linear reduction in 

tensile stress results 

in an exponential 

increase in fatigue life 

(Number of Load 

Cycles). The graph 

(Figure 1-4) shows 

that a 38 ksi (262 

MPa) reduction in 

stress (32%) results in 

Figure 1-4 Typical Stress vs Load Cycles 

a 150,000 cycle life increase (300%). 

S H O T P E E N I N G 

R E S I D U A L 

S T R E S S 

The residual stress generated by 

shot peening is of a compressive 

nature. This compressive stress 

offsets or lowers applied tensile 

stress. Quite simply, less (tensile) 

stress equates to longer part life. 

A typical shot peening stress profile 

is depicted in Figure 1-5. 

Maximum Compressive Stress – 

This is the maximum value of 

compressive stress induced. It is 

normally just below the surface. 

As the magnitude of the maximum 

Figure 1-5 Typical Shot Peening Residual Stress Profile 

compressive stress increases so does the resistance to fatigue cracking. 

Depth of Compressive Layer – This is the depth of the compressive layer resisting crack growth. The layer 

depth can be increased by increasing the peening impact energy. A deeper layer is generally desired for 

crack growth resistance. 

Surface Stress – This magnitude is usually less than the Maximum Compressive Stress. 



THEORY 

7


THEORY 

8 

SUMMATION OF APPLIED 

AND RESIDUAL STRESS 

When a component is shot peened and subjected to 

an applied load, the surface of the component 

experiences the net stress from the applied load and 

shot peening residual stress. Figure 1-6 depicts a 

bar with a three-point load that creates a bending 

stress at the surface. 

The diagonal dashed line is the tensile stress created 

from the bending load. The curved dashed line is the 

(residual) compressive stress from shot peening. 

The solid line is the summation of the two showing a 

significant reduction of tensile stress at the surface. 

Shot peening is highly advantageous for the following two conditions. 

• Stress risers • High strength materials 

Stress risers may consist of radii, notches, cross holes, grooves, keyways, etc… Shot peening induces a 

high magnitude, localized compressive stress to offset the stress concentration factor created from these 

geometric changes. 

Shot peening is ideal for high strength materials. Compressive stress is directly correlated to a material’s 

tensile strength. The higher the tensile strength, the more compressive stress that can be induced. Higher 

strength materials have a more rigid crystal structure. This crystal lattice can withstand greater degrees of 

strain and consequently can store more residual stress. 

A p p l i c a t i o n C a s e S t u d y 

NASA LANGLEY CRACK GROWTH STUDY 

Engineers at NASA performed a study on crack growth rates of 2024-T3 aluminum with and without shot 

peening. The samples were tested with an initial crack of 0.050" (1.27 mm) and then cycle tested to failure. It 

should be noted that the United States Air Force damage tolerance rogue flaw is 0.050" (1.27 mm). 

It was found that crack growth was significantly delayed when shot peening was included. As the following 

results demonstrate, at a 15 ksi (104 MPa) net stress condition the remaining life increased by 237%. At a 20 

ksi (138 MPa) net stress condition the remaining life increased by 81%. 

This test reflects conditions that are harsher than real world conditions. Real world conditions would generally 

not have initial flaws and should respond with better fatigue life improvements at these stress levels. 

NON-SHOT PEENED TEST RESULTS 

Net Number Average 

Stress Of Tests Life Cycles 

15 ksi 2 75,017 

20 3 26,029 

Note on sample preparation: A notch was placed in the surface via the EDM process. The samples were loaded 

in fatigue until the crack grew to ~ 0.050" (1.27 mm). If samples were shot peened, they were peened after the 

initial crack of 0.050" (1.27 mm) was generated. This was the starting point for the above results. [Ref 1.1] 


Figure 1-6 Resultant Stress in a Shot Peened 

Beam with an External Load Applied 

SHOT PEENED TEST RESULTS 

Net Number Average Percent 

Stress Of Tests Life Cycles Increase 

15 ksi 2 253,142 237% 

20 3 47,177 81%

D E P T H O F R E S I D U A L 

S T R E S S 

The depth of the compressive layer is influenced by 

variations in peening parameters and material 

hardness [Ref 1.2]. Figure 1-7 shows the relationship 

between the depth of the compressive layer and the 

shot peening intensity for five materials: steel 31 HRC, 

steel 52 HRC, steel 60 HRC, 2024 aluminum and 

titanium 6Al-4V. Depths for materials with other 

hardness values can be interpolated. 

S H O T P E E N I N G M E D I A 

Media used for shot peening (see also Chapter 12) 

consists of small spheres of cast steel, conditioned 

cut wire (both carbon and stainless steel), ceramic or 

glass materials. Most often cast or wrought carbon 

steel is employed. Stainless steel media is used in 

Figure 1-7 Depth of Compression 

vs. Almen Arc Height 

applications where iron contamination on the part surface is of concern. 

Carbon steel cut wire, conditioned into near round 

shapes, is being specified more frequently due to its 

uniform, wrought consistency and great durability. It 

is available in various grades of hardness and in much 

tighter size ranges than cast steel shot. 

Glass beads are also used where iron contamination is 

of concern. They are generally smaller and lighter 

than other media and can be used to peen into sharp 

radii of threads and delicate parts where very low 

intensities are required. 

EFFECT O F S H OT 

HARDNESS 

It has been found that the hardness of the shot will 

influence the magnitude of compressive stress 

(Figure 1-8). The peening media should be at least as 

hard or harder than the parts being peened unless 

Figure 1-8 Peening 1045 Steel (Rc 50+) [Ref 1.3] 

surface finish is a critical factor. For a large number of both ferrous and nonferrous parts, this criterion is 

met with regular hardness steel shot (45-52 HRC). 

The increased use of high strength, high hardness steels (50 HRC and above) is reflected in the use of 

special hardness shot (55-62 HRC). 

REFERENCES: 

1.1 Dubberly, Everett, Matthews, Prabhakaran, Newman; The Effects of Shot and Laser Peening on Crack Growth and Fatigue Life in 2024 

Aluminum Alloy and 4340 Steel, US Air Force Structural Integrity Conference, 2000 

1.2 Fuchs; Shot Peening Stress Profiles 

1.3 Lauchner, WESTEC Presentation March 1974, Northrup Corporation; Hawthorne, California 



THEORY 

9

C H A P T E R T W O 


10 

H I G H S T R E N G T H S T E E L S 

The residual compressive stress induced by shot peening is a percentage of the ultimate tensile strength 

and this percentage increases as the strength/hardness increases. Higher strength/hardness metals 

tend to be brittle and sensitive to surface notches. These conditions can be overcome by shot peening 

permitting the use of 

high strength metals in 

fatigue prone 

applications. Aircraft 

landing gear are often 

designed to strength 

levels of 300 ksi (2068 

MPa) that incorporate 

shot peening. Figure 2- 

1 shows the relationship 

between shot peening 

and use of higher 

strength materials. 

Without shot peening, 

optimal fatigue 

Figure 2-1 Fatigue Strength vs. Ultimate Tensile Strength 

properties for machined 

steel components are obtained at approximately 30 HRC. At higher strength/hardness levels, materials 

lose fatigue strength due to increased notch sensitivity and brittleness. With the addition of compressive 

stresses, fatigue strength increases proportionately to increasing strength/hardness. At 52 HRC, the 

fatigue strength of the shot peened specimen is 144 ksi (993 MPa), more than twice the fatigue strength 

of the unpeened, smooth specimen [Ref 2.1]. 

Typical applications that take advantage of high strength/hardness and excellent fatigue properties with 

shot peening are impact wrenches and percussion tools. In addition, the fatigue strength of peened 

parts is not impaired by shallow scratches that could otherwise be detrimental to unpeened high 

strength steel [Ref 2.2]. 


C A R B U R I Z E D S T E E L S 

Carburizing and carbonitriding are heat treatment processes that result in very hard surfaces. They are 

commonly 55-62 HRC. The benefits of shot peening carburized steels are as follows: 

• High magnitudes of compressive stress of ~ 200 ksi (1379 MPa) or greater offer 

excellent fatigue benefits 

• Carburizing anomalies resulting from surface intergranular oxidation are reduced. 

Shot hardness of 55-62 HRC is recommended for fully carburized and carbonitrided parts if maximum 

fatigue properties are desired. 


HIGH PERFORMANCE CRANKSHAFTS 

Crankshafts for 4-cylinder 

high performance engines 

were failing prematurely 

after a few hours running on 

test at peak engine loads. 

Testing proved that gas 

carburizing and shot peening 

the crankpins gave the best 

fatigue performance (Figure 

2-2). Results from nitriding 

and shot peening also 

demonstrated favorable 

results over the alternative 

to increase the crankpin 

diameter [Ref 2.3]. 

Figure 2-2 Comparison of Shot Peened 

Nitrided & Gas Carburized Crank Pins 

D E C A R B U R I Z A T I O N 

Decarburization is the reduction in surface carbon content of a ferrous alloy during thermal processing. 

It has been shown that decarburization can reduce the fatigue strength of high strength steels (240 ksi, 

1650 MPa or above) by 70-80% and lower strength steels (140-150 ksi, 965-1030 MPa) by 45-55% 

[Refs 2.4, 2.5 and 2.6]. 

Decarburization is a surface phenomenon not particularly related to depth. A depth of 0.003 inch 

decarburization can be as detrimental to fatigue strength as a depth of 0.030 inch [Refs 2.4, 2.5 and 2.6]. 




11



12 

Shot peening has proven to be effective in restoring most of the fatigue strength lost due to decarburization 

[Ref 2.7]. Because the decarburized layer is not easily detectable on quantities of parts, peening can 

insure the integrity of the parts if decarburization is suspected. If a gear that is intended to have a high 

surface hardness (58+ HRC) exhibits unusually heavy dimpling after peening, decarburization should 

be suspected. 

Decarburization is often accompanied with the undesirable metallurgical condition of retained austenite. 

By cold working the surface, shot peening reduces the percentage of retained austenite. 


REDUCTION OF RETAINED AUSTENITE - 5120 CARBURIZED STEEL, 

SHOT PEENED AT 0.014" (0.36 mm) A INTENSITY 

Retained Austenite 

(Volume %) 

Depth (inches) Depth (mm) Unpeened Peened 

0.0000 0.00 5 3 

0.0004 0.01 7 4 

0.0008 0.02 14 5 

0.0012 0.03 13 6 

0.0016 0.04 14 7 

0.0020 0.05 14 7 

0.0024 0.06 15 8 

0.0028 0.07 15 9 

0.0039 0.10 15 10 

0.0055 0.14 12 10 

[Ref 2.8] 

A U S T E M P E R E D D U C T I L E I R O N 

Improvements in austempered ductile iron (ADI) have allowed it to replace steel forgings, castings, and 

weldments in some engineering applications. ADI has a high strength-to-weight ratio and the benefit of 

excellent wear capabilities. ADI has also replaced aluminum in certain high strength applications as it is at 

least 3 times stronger and only 2.5 times more dense. With the addition of shot peening, the allowable 

bending fatigue strength of ADI can be increased up to 75%. This makes certain grades of ADI with shot 

peening comparable to case-carburized steels for gearing applications [Ref 2.9]. 

C A S T I R O N 

There has been an increased demand in recent years for nodular cast iron components that are capable of 

withstanding relatively high fatigue loading. Cast iron components are often used without machining in 

applications where the cast surface is subject to load stresses. The presence of imperfections at casting 

surfaces in the form of pinholes, dross or flake graphite can considerably reduce the fatigue properties of 

unmachined pearlitic nodular irons. The unnotched fatigue limit may be reduced by as much as 40%, 

depending on the severity of the imperfections at the casting surface. 


Shot peening can significantly improve properties when small cast-surface imperfections are present. One 

application is diesel engine cylinder liners. At the highest shot peening intensity used in the tests, the 

fatigue limit was 6% below that of fully machined fatigue specimens. This compares to a reduction of 20% 

for specimens in the as-cast unpeened condition. Visually, the peening on the as-cast surface has a 

polishing effect leaving the appearance of smoothing the rougher as-cast surface [Ref 2.10]. 

A L U M I N U M A L L O Y S 

Traditional high strength aluminum alloys (series 2000 & 7000) have been used for decades in the aircraft 

industry because of their high strength-to-weight ratio. The following aluminum alloys have emerged with 

increasing use in critical aircraft/aerospace applications and respond equally well to shot peening: 

• Aluminum Lithium Alloys (Al-Li) 

• Isotropic Metal Matrix Composites (MMC) 

• Cast Aluminum (Al-Si) 


Al 7050-T7651 HIGH STRENGTH ALUMINUM 

Fatigue specimens were 

prepared from high strength 

Al 7050-T7651. All four sides 

of the center test portion 

were shot peened. Fatigue 

tests were conducted under 

a four-point reversed 

bending mode (R = -1). The 

S-N curve of the shot peened 

versus non-shot peened 

alloy is shown in Figure 2-3. 

It was found that shot 

peening improved the 

fatigue endurance limit by 

Figure 2-3 S-N Curves for Shot Peened 

approximately 33%. Even in 

a regime where the stress 

Aluminum Alloy 7050-T7651 

ratio is between the yield strength and the endurance limit, the fatigue strength increased by a factor of 

2.5 to almost 4 [Ref 2.11]. 




13



14 

T I T A N I U M 

High Cycle Fatigue (HCF) - HCF of 

titanium is illustrated by Figure 2-4 that 

compares the capabilities of titanium 

alloy connecting rods for a high 

performance European sports car. The 

rods are manufactured using various 

processes. With shot peening, the 

fatigue limit was increased by 

approximately 20% while weight was 

reduced by some 40% as compared to 

steel connecting rods [Ref 2.12]. 

Low Cycle Fatigue (LCF) - As is typical 

with other metals, the fatigue response 

with shot peening increases with 

higher cycle fatigue. Higher cycle 

fatigue would be associated with lower 

stresses whereas lower cycle fatigue 

would be associated with higher stress 

levels. This is demonstrated 

graphically in the S-N Curve (Figure 1-4) 

and also Figure 2-5. 

Figure 2-5 shows the results of shot 

peening titanium dovetail slots on a 

rotating engine component [Ref 2.13]. 

There are two baseline load curves that 

are not shot peened. When shot 

peening is applied, the base line curve 

that initially had more cycles to failure 

responded significantly better. Note 

that improvements in fatigue life are on 

an exponential basis. 

Figure 2-4 Comparison of Fatigue Strengths for Polished 

and Shot Peened Titanium Ti6A14V 

Figure 2-5 LCF Benefits from Shot Peening Notched Ti8-1-1 

The most common application of LCF for titanium is the rotating turbine engine hardware (discs, spools, 

& shafts) with the exception of blades. These components are shot peened to increase durability. Each 

takeoff and landing is considered one load cycle. 

M A G N E S I U M 

Magnesium alloys are not commonly used in fatigue applications. However, when used for the benefit of 

weight reduction, special peening techniques can be employed to achieve 25 - 35% improvement in 

fatigue strength. 


P O W D E R M E T A L L U R G Y 

Optimized peening parameters have been shown to raise the endurance limit of sintered steel PM alloys by 

22% and the fatigue life by a factor of 10. [Ref. 2.14] Automotive components such as gears and connecting 

rods are candidates for PM with shot peening. Shot peening is most effective on higher density PM parts 

such as forged powder metal components. 

Surface densification by shot peening can increase the fatigue limit significantly, especially in the case of 

bending. The surface densification also assists in the closing of surface porosity of PM components for 

sealing and other engineering applications. 

Pressed and sintered ferrous powder materials are in increasing demand as the PM industry has grown into 

applications involving more highly stressed components. Ancorsteel 1000B with 2% copper and 0.9% 

graphite had an endurance limit of 35 ksi (240 MPa) when tested without shot peening. Shot peening the 

test specimens increased the endurance limit 16% to 40.5 ksi (280 MPa) [Ref 2.16]. 

REFERENCES: 


HIGH DENSITY POWDER METAL GEARS 

As part of a project sponsored by the German Federal Ministry of Education and Research, powdered metal 

alloys were tested for suitability in gearing applications. A MSP4.0Mo-0.1Nb powdered metal gear was 

tested against a reference machined 20MnCr5 case hardened steel. Tooth root load carrying capacity tests 

produced the following fatigue strength results (at 2 million load cycles). Note: the reference wrought 

steel’s fatigue strength was defined as 100% 

• Reference: Unpeened 20MnCr5 (wrought steel): 100% 

• Unpeened MSP4.0Mo-0.1Nb (powdered metal): 82% 

• Shot Peened MSP4.0Mo-0.1Nb (powdered metal): 109% 

The tests proved that the unpeened powdered metal had a fatigue strength 18% less than the wrought 

steel gear material. The shot peened powdered metal’s fatigue strength was 9% higher than the reference 

wrought steel [Ref 2.15]. 

2.1 Horger; Mechanical and Metallurgical Advantages of Shot Peening – Iron Age Reprint 1945 

2.2 Hatano and Namitki; Application of Hard Shot Peening to Automotive Transmission Gears, Special Steel Research Laboratory, Daido Steel 

Company, Ltd., Japan. 

2.3 Challenger; Comparison of Fatigue Performance Between Engine Crank Pins of Different Steel Types and Surface Treatments, Lucas Research 

Center, Solihull, England, July 1986 

2.4 Properties and Selection, Metals Handbook, Eighth Edition, Vol. 1, pp. 223-224. 

2.5 Jackson and Pochapsky; The Effect of Composition on the Fatigue Strength of Decarburized Steel, Translations of the ASM, Vol. 39, pp. 45-60. 

2.6 Bush; Fatigue Test to Evaluate Effects of Shot Peening on High Heat Treat Steel - Lockheed Report No. 9761. 

2.7 Gassner; Decarburization and Its Evaluation by Chord Method, Metal Progress, March 1978, pp. 59-63. 

2.8 Internal Metal Improvement Co. Memo 

2.9 Keough, Brandenburg, Hayrynen; Austempered Gears and Shafts: Tough Solutions, Gear Technology March/April 2001, pp. 43-44. 

2.10 Palmer; The Effects of Shot Peening on the Fatigue Properties of Unmachined Pearlitic Nodular Graphite Iron Specimens Containing Small 

Cast Surface Imperfections, BCIRA Report #1658, The Casting Development Centre, Alvechurch, Birmingham, UK. 

2.11 Oshida and Daly; Fatigue Damage Evaluation of Shot Peened High Strength Aluminum Alloy, Dept. of Mechanical and Aerospace Engineering, 

Syracuse University, Syracuse, NY 

2.12 Technical Review, Progress in the Application of Shot-Peening Technology for Automotive Engine Components, Yamaha Motor Co., Ltd., 1998. 

2.13 McGann and Smith; Notch Low Cycle Fatigue Benefits from Shot Peening of Turbine Disk Slots. 

2.14 Sonsino, Schlieper, Muppman; How to Improve the Fatigue Properties of Sintered Steels by Combined Mechanical and Thermal Surface 

Treatments, Modern Developments in Powder Metallurgy, Volume 15 - 17, 1985. 

2.15 Link, Kotthoff; Suitability of High Density Powder Metal Gears for Gear Applications; Gear Technology, January/February 2001. 

2.16 O’Brian; Impact and Fatigue Characterization of Selected Ferrous P/M Materials, Annual Powder Metallurgy Conference, Dallas, TX. May 1987. 




15

C H A P T E R T H R E E 

MANUFA CTURING PROCESSES 

16 

E F F E C T O N F A T I G U E L I F E 

Manufacturing processes have significant effects on fatigue properties of metal parts. The effects can be 

either detrimental or beneficial. Detrimental processes include welding, grinding, abusive machining, metal 

forming, etc… These processes leave the surface in residual tension. The summation of residual tensile 

stress and applied loading stress accelerates fatigue failure as shown in Figure 1-6. 

Beneficial manufacturing processes include surface hardening as it induces some residual compressive 

stress into the surface. Honing, polishing and burnishing are surface enhancing processes that remove 

defects and stress raisers from manufacturing operations. Surface rolling induces compressive stress but is 

primarily limited to cylindrical geometries. Shot peening has no geometry limitations and produces results 

that are usually the most economical. 

The effect of residual stress on fatigue life is demonstrated in the following example. A test by an airframe 

manufacturer on a wing fitting showed the initiation of a crack at just 60% of predicted life. The flaw was 

removed and the same area of the part shot peened. The fitting was then fatigue tested to over 300% life 

without further cracking even with reduced cross sectional thickness [Ref 3.1]. 

W E L D I N G 

The residual tensile stress from welding is created because the weld consumable is applied in its molten state. 

This is its hottest, most expanded state. It then bonds to the base material, which is much cooler. The weld 

cools rapidly and attempts to shrink during the cooling. Since it has already bonded to the cooler, stronger 

base material it is unable to shrink. The net result is a weld that is essentially being "stretched" by the base 

material. The heat affected zone is usually most affected by the residual stress and hence where failure will 

usually occur. Inconsistency in the weld 

filler material, chemistry, weld geometry, 

porosity, etc… act as a stress risers for 

residual and applied tensile stress to 

initiate fatigue failure. 

As shown in Figure 3-1, shot peening is 

extremely beneficial in reversing the 

residual stress from welding that tends to 

cause failure in the heat affected zone 

from a tensile to a compressive state. 

Figure 3-1 demonstrates a number of 

interesting changes in residual stress 

from welding, thermal stress relieving, 

and shot peening [Ref 3.2]. Tensile 

stresses generated from welding are 

Figure 3-1 Residual Stresses from Welding 

additive with applied load stresses. This combined stress will accelerate failure at welded connections. 

When the weld is stress relieved at 1150 °F (620 °C) for one hour, the tensile stress is reduced to almost zero. 

This reduction of tensile stress will result in improved fatigue properties. 


If the weld is shot peened (rather than stress relieved) there is a significant reversal of residual stress from 

tensile to compressive. This will offer significant resistance to fatigue crack initiation and propagation. 

Figure 3-1 shows the optimal manufacturing sequence for welding is to stress relieve and then shot 

peen. The stress relieving process softens the weld such that inducing a deeper layer of compressive 

stress becomes possible. 


FATIGUE OF OFFSHORE STEEL STRUCTURES 

A Norwegian research program 

concluded that the combination of Steel Condition 

Fatigue Strength 

At 1,000,000 Cycles 

weld toe grinding and shot peening 

Base Material ~ 50 ksi (340 MPa) 

gave the largest improvement in 

Weld Toe Ground & Peened ~ 44 ksi (300 MPa) 

the structure life. This corresponds Weld Toe Ground (only) ~ 26 ksi (180MPa) 

to more than a 100% increase in 

the as-welded strength at one 

As Welded (only) ~ 20 ksi (140MPa) 

million cycles [Ref 3.3]. Other research shows that the improvement in weld fatigue strength from shot 

peening increases in proportion to the yield strength of the parent metal. 

The American Welding Society (AWS) Handbook cautions readers to consider residual tensile stresses from 

welding if the fabrication is subject to fatigue loading as described in the following statement. "Localized 

stresses within a structure may result entirely from external loading, or there may be a combination of 

applied and residual stresses. Residual stresses are not cyclic, but they may augment or detract from 

applied stresses depending on their respective sign. For this reason, it may be advantageous to induce 

compressive residual stress in critical areas of the weldment where cyclic applied stresses are expected". 

The use of the shot peening process to improve resistance to fatigue as well as stress corrosion cracking in 

welded components is recognized by such organizations as: 

• American Society of Mechanical Engineers [Ref. 3.4] 

• American Bureau of Shipping [Ref. 3.5] 

• American Petroleum Institute [Ref. 3.6] 

• National Association of Corrosion Engineers [Ref. 3.7] 


TURBINE ENGINE HP COMPRESSOR ROTORS 

Two leading companies in the manufacture of jet turbine engines jointly manufacture high pressure 

compressor rotors. Separate pieces are machined from forged titanium (Ti 4Al-6V) and then welded 

together. Testing produced the following results: 

As Welded 4,000 cycles* 

Welded and polished 6,000 cycles 

Welded and peened 16,000 cycles 


* In aircraft engine terminology one cycle 

equals the ramp up required for one take-off of 

the aircraft for which the engine is configured. 

Initially, shot peening was used as additional "insurance" from failure. After many years of failure 

free service, coupled with innovations in shot peening controls, shot peening has been incorporated 

as a full manufacturing process in engine upgrades [Ref 3.8]. 



17



18 

G R I N D I N G 

Typically, grinding induces residual tensile stress 

as a result of localized heat generated during 

the process. The metal in contact with the 

abrasive medium heats locally and attempts to 

expand. The heated material is weaker than the 

surrounding metal and yields in compression. 

Upon cooling the yielded metal attempts to 

contract. This contraction is resisted by the 

surrounding metal resulting in residual tensile 

stress. Residual tensile stress of any magnitude 

will have a negative effect on fatigue life and 

resistance to stress corrosion cracking. 

Figure 3-2 graphically depicts residual tensile 

stress generated from various grinding processes [Ref 3.9]. A 1020, 150-180 BHN carbon steel (with and 

without weld) was ground abusively and conventionally. Figure 3-2 shows that the grinding processes 

resulted in high surface tension with the abusive grind having a deeper (detrimental) layer of residual tension. 

Shot peening after grinding will reverse the residual stress state from tensile to compressive. The beneficial 

stress reversal is similar to that from shot peening welded regions in a state of tension. 

Figure 3-3 Plating Micro-Cracks 

P L A T I N G 

Many parts are shot peened prior to chrome and electroless 

nickel plating to counteract the potential harmful effects on 

fatigue life. Fatigue deficits from plating may occur from the 

micro-cracking in the brittle surface, hydrogen embrittlement 

or residual tensile stresses. 

Figure 3-3 is a 1200x SEM photograph showing a network of 

very fine cracks that is typical of hard chrome plating [Ref 3.10]. 

Under fatigue loading, the micro-cracks can propagate into the 

base metal and lead to fatigue failure. 

When the base metal is shot peened, the potential for fatigue 

crack propagation into the base metal from the plating is 

dramatically reduced. Figure 3-4 illustrates this concept and 

Figure 3-4 Compressive Stress Resists assumes dynamic loading on a component. 

Micro-Crack Growth 

The graphic on the left shows the micro-cracking propagating 

into the base material. When shot peened, the graphic on the right shows the compressive layer preventing 

the micro-cracking from propagating into the base material. 

Shot peening prior to plating is recommended on cyclically loaded parts to enhance fatigue properties. For 

parts that require unlimited life under dynamic loads, federal specifications QQ-C-320 and MIL-C-26074 call for 

shot peening of steel parts prior to chrome or electroless nickel-plating. Other hard plating processes such as 

electrolytic nickel may also lower fatigue strength. 


Figure 3-2 Residual Stresses from Grinding

A N O D I Z I N G 

Hard anodizing is another application in which shot peening improves fatigue resistance of coated 

materials. Benefits are similar to those for plating providing the peening is performed to the base 

material before anodizing. 


ANODIZED ALUMINUM RINGS 

Aluminum (AlZnMgCu 0.5) rings with external teeth were tested for comparison purposes with 

anodizing and shot peening. The rings had an outside diameter of ~ 24" (612 mm) and a tensile 

strength of ~ 71 ksi (490 MPa). The (hard) anodizing layer was ~ 0.0008" (0.02 mm) thick. 

Bending fatigue tests were 

conducted to find the load 

to cause a 10% failure 

probability at one million 

cycles. The following table 

shows the results [Ref 3.11]. 

P L A S M A S P R A Y 

Plasma spray coatings are primarily used in applications that require excellent wear resistance. Shot 

peening has proven effective as a base material preparation prior to plasma spray applications that are 

used in cyclic fatigue applications. Shot peening has also been used after the plasma spray application to 

improve surface finish and close surface porosity. 

E L E C T R O - D I S C H A R G E M A C H I N I N G ( E D M ) 

EDM is essentially a "force-free" spark erosion 

process. The heat generated to discharge 

molten metal results in a solidified recast layer 

on the base material. This layer can be brittle 

and exhibit tensile stresses similar to those 

generated during the welding process. Shot 

peening is beneficial in restoring the fatigue 

debits created by this process. In Figure 3-5 

the effect of shot peening on electro-chemical 

machined (ECM), electro-discharge machined 

(EDM) and electro-polished (ELP) surfaces is 

shown [Ref 3.12]. Figure 3-5 should be viewed 

in a clockwise format. ECM, EDM, & ELP 

fatigue strengths are compared with and 

without shot peening. 

Shot Peened Hard Anodized Load (10% Failure) 

No No 6744 lb / 30 KN 

Yes No 9218 lb / 41 KN 

No Yes 4496 lb / 20 KN 

Yes Yes 10,791 lb / 48 KN 


Figure 3-5 High Cycle Fatigue Behavior 

of Inconel 718 



19



20 

E L E C T R O - C H E M I C A L M A C H I N I N G ( E C M ) 

Electro-chemical machining is the controlled dissolution of material by contact with a strong chemical 

reagent in the presence of an electric current. A reduction in fatigue properties is attributed to surface 

softening (the rebinder effect) and surface imperfections left by preferential attack on grain boundaries. 

A shot peening post treatment more than restores fatigue properties as shown in Figure 3-5 [Ref 3.12]. 


DIAPHRAGM COUPLINGS 

Metal diaphragm couplings are often used in turbo-machinery applications. These couplings accommodate 

system misalignment through flexing. This flexing, or cyclic loading, poses concerns for fatigue failures. 

Researchers concluded that the ECM process produces parts that are geometrically near-perfect. However, 

they found under scanning electron microscope observation that small cavities sometimes developed on 

the surface as a result of ECM. These cavities apparently generated stress concentrations that lead to 

premature failures. Shot peening after ECM was able to overcome this deficiency and has significantly 

improved the endurance limit of the diaphragm couplings [Ref 3.13 & 3.14]. 

REFERENCES: 


3.2 Molzen, Hornbach; Evaluation of Welding Residual Stress Levels Through Shot Peening and Heat Treating, AWS Basic Cracking 

Conference; Milwaukee, WI; July 2000 

3.3 Haagensen; Prediction of the Improvement in Fatigue Life of Welded Joints Due to Grinding, TIG Dressing, Weld Shape Control and Shot 

peening." The Norwegian Institute of Technology, Trondheim, Norway. 

3.4 McCulloch; American Society of Mechanical Engineers, Letter to H. Kolin, May 1975. 

3.5 Stern; American Bureau of Shipping, Letter to G. Nachman, July 1983. 

3.6 Ubben; American Petroleum Institute, Letter to G. Nachman, February 1967. 

3.7 N.A.C.E Standard MR-01-75, Sulfide Stress Cracking Resistant Metallic Material for Oilfield Equipment, National Association of Corrosion Engineers. 


3.9 Molzen, Hornbach; Evaluation of Welding Residual Stress Levels Through Shot Peening and Heat Treating, AWS Basic Cracking Conference; 

Milwaukee, WI; July 2000 

3.10 Metallurgical Associates, Inc; "Minutes" Vol.5 No.1, Winter 1999; Milwaukee, WI 


3.12 Koster, W.P., Observation on Surface Residual Stress vs. Fatigue Strength, Metcut Research Associates, Inc., Cincinnati, Ohio. 

Bulletin 677-1, June 1977 

3.13 Calistrat; Metal Diaphragm Coupling Performance, Hydrocarbon Processing, March 1977 

3.14 Calistrat; Metal Diaphragm Coupling Performance, 5th Turbomachinery Symposium, Texas A&M University, October 1976 


B E N D I N G 

F A T I G U E 

Bending fatigue is the most 

common fatigue failure mode. This 

failure mode responds well to shot 

peening because the highest 

(tensile) stress is at the surface. 

Figure 4-1 shows a cantilever beam 

under an applied bending load. 

The beams deflection causes the 

top surface to stretch putting it in a 

state of tension. Any radii or 

geometry changes along the top 

surface would act as stress risers. 

Figure 4-1 Highest Stress at Surface 

Fully reversed bending involves components that cycle through tensile and compressive load cycles. This 

is the most destructive type of fatigue loading. Fatigue cracks are initiated and propagated from the 

tensile portion of the load cycle. 

G E A R S 

Shot peening of gears is a very common application. Gears of any 

size or design are mainly shot peened to improve bending fatigue 

properties in the root sections of the tooth profile. The meshing of 

gear teeth is similar to the cantilever beam example. The load 

created from the tooth contact creates a bending stress in the root 

area below the point of contact (Figure 4-3). 

Figure 4-2 Ring & Pinion 

Gears are frequently shot peened after through hardening or surface 

hardening. 

Gear Assembly 

Increased 

surface 

hardness results in proportional increases in 

compressive stress. Maximum residual 

compression from carburized and shot peened 

gears can range from 170-230 ksi (1170-1600 

MPa) depending on the carburizing treatment 

and shot peening parameters (Figure 4-4). It is 

common to use hard shot (55-62 HRC) when 

shot peening carburized gears. However, 

reduced hardness shot (45-52 HRC) may be 

used when carburized surfaces require less 

Figure 4-3 Polarized View of Applied Gear Stresses 


C H A P T E R F O U R 

BENDING FA T IGUE 

21



22 

disruption of the tooth flank surface. 

The amount of compressive stress will 

be ~ 50% of that which hard shot will 

induce. 

The optimal way to induce resistance 

to pitting fatigue near the gear tooth 

pitch line is to induce a compressive 

stress followed by a lapping, honing, or 

isotropic finishing process. Care must 

be taken to not remove more than 10% 

of the shot peening layer. Processes 

that refine the surface finish of shot 

peening dimples allow the contact load 

Figure 4-4 Typical Carburized Gear Residual Stress Profile 

to be distributed over a larger surface area reducing contact stresses. 

Metal Improvement Co. offers a shot peening and superfinishing process called C.A.S.E. SM that has increased 

pitting fatigue resistance of gears by 500%. Please see Chapter 11 for additional information and photomicrographs 

on this process. 

Increases in fatigue strength of 30% or more at 1,000,000 cycles are common in certain gearing 

applications. The following organizations/specifications allow for increases in tooth bending loads when 

controlled shot peening is implemented. 

• Lloyds Register of Shipping: 20% increase [Ref 4.2] 

• Det Norske Veritas: 20% increase [Ref 4.3] 

• ANSI/AGMA 6032-A94 Marine Gearing Specification: 15% increase 

C O N N E C T I N G R O D S 

Connecting rods are excellent examples of 

metal components subjected to fatigue 

loading as each engine revolution results 

in a load cycle. The critical failure areas 

on most connecting rods are the radii on 

either side of the I-beams next to the 

large bore. Figure 4-5 shows a finite 

element stress analysis plot with the 

maximum stress areas shown in red. 

The most economical method of shot 

peening is to peen the as-forged, as-cast 

or powdered metal rod prior to any 

Figure 4-5 Finite Element Stress Plot of a Connecting Rod 

machining of the bores or faces. This eliminates masking operations that will add cost. Rougher 

surfaces in compression have better fatigue properties than smooth surfaces in tension (or without 

residual stress) such that most peened surfaces do not require prior preparation or post operations. 


C R A N K S H A F T S 

In most cases, all radii on a crankshaft are shot peened. These 

include the main bearing journals and crankpin radii as shown in 

Figure 4-6. The most highly stressed area of a crankshaft is the 

crank pin bearing fillet. The maximum stress area is the bottom 

side of the pin fillet when the engine fires as the pin is in the top 

dead center position (Figure 4-6). It is common for fatigue 

cracks to initiate in this pin fillet and propagate through the web 

of the crankshaft to the adjacent main bearing fillet causing 

catastrophic failure. 

Experience has shown shot peening to be effective on forged steel, 

cast steel, nodular iron, and austempered ductile iron crankshafts. Fatigue strength increases of 10 to 30% are 

allowed by Norway’s Det Norske Veritas providing fillets are shot peened under controlled conditions [Ref 4.5]. 


DIESEL ENGINE CRANKSHAFTS 

REFERENCES: 

4.1 Figure 4-2, Unigraphics Solutions, Inc. website (www.ugsolutions / www.solid-edge.com), June 2000 

4.2 Letter to W.C. Classon, Lloyds Register of Shipping, May 1990 

4.3 Sandberg; Letter to Metal improvement Company, Det Norske Veritas, September 1983 


4.5 Sandberg; Letter to Metal improvement Company, Det Norske Veritas, September 1983 


4.7 FAA Issues AD on TFE73, Aviation week & Space Technology; April 22, 1991 


Figure 4-6 Crankshaft Schematic 

Four point bending fatigue tests were carried out on test pieces from a diesel engine crankshaft. The 

material was Armco 17-10 Ph stainless steel. The required service of this crankshaft had to exceed one 

hundred million cycles. Fatigue strength of unpeened and shot peened test pieces were measured at one 

billion cycles. The fatigue strength for the unpeened material was 43 ksi (293 MPa) versus 56 ksi (386 

MPa) for the peened material. This is an increase of ~ 30% [Ref 4.6]. 


TURBINE ENGINE DISKS 

In 1991 the Federal Aviation Authority issued an 

airworthiness directive that required inspection for cracks 

in the low pressure fan disk. Over 5,000 engines were in 

use on business jets in the United States and Europe. 

The FAA required that engines that did not have lance 

(shot) peening following machining in the fan blade 

dovetail slot be inspected. Those engines having fan 

disks without lance peening were required to reduce 

service life from 10,000 to 4,100 cycles (takeoffs and 

landings). Disks that were reworked with lance peening 

per AMS 2432 (Computer Monitored Shot Peening) prior 

Figure 4-7 Lance Peening of Fan Disk 

to 4,100 cycles were granted a 3,000 cycle extension [Ref 4.7]. A typical lance peening operation of a fan 

disk is shown in Figure 4-7. See also Chapter 11 Internal Surfaces & Bores. 



23

C H A P T E R F I V E 

T ORSIONAL F A T IGUE 

24 

T O R S I O N A L 

F A T I G U E 

Torsional fatigue is a failure mode that 

responds well to shot peening because 

the greatest (tensile) stress occurs at 

the surface. Torsional loading creates 

stresses in both the longitudinal and 

perpendicular directions such that the 

maximum tensile stress is 45° to 

longitudinal axis of the component. 

Figure 5-1 depicts a solid bar loaded in 

pure torsion with a crack depicting reversed torsional loading. 

Lower strength materials tend to fail from torsional fatigue in the shear plane perpendicular to the longitudinal 

axis. This is because they are weaker in shear than in tension. Higher strength materials tend to fail at 45° to 

the longitudinal axis because they are weaker in tension than in shear. 

Figure 5-2 Compression 

Spring Assembly 

The spring wire analyzed in Figure 5-3 

was a 0.25" (6.35 mm) diameter 

Chrome-Silicon material with an 

ultimate tensile strength (UTS) of 260 

ksi (1793 MPa). The residual tensile 

stress at the ID after coiling was ~ 70 

ksi (483 MPa) and is the primary 

reason for failure at 80,000 load 

cycles [Ref 5.2]. 

Shot peening induced a reversal of 

residual stress to a maximum 

compressive stress of ~ 150 ksi (1035 

MPa). This is 60% of UTS of the wire 

and resulted in fatigue life in excess of 

500,000 load cycles without failure. 

C O M P R E S S I O N S P R I N G S 

Compression springs are subject to high cycle fatigue and are one of the 

more common shot peening applications. The spring wire twists 

allowing the spring to compress creating a torsional stress. In addition 

to operating in high cycle fatigue conditions, the coiling process induces 

detrimental tensile stress at the inner diameter (ID) of the spring. 

Figure 5-3 demonstrates the residual stress after coiling and shot 

peening. 


Figure 5-1 Torsional Loading 

Figure 5-3 Coil Spring ID Residual Stress 

With and Without Shot Peen

It is quite common to perform a baking operation after shot peening of springs. The baking operation is 

used as a stabilizing process in the manufacture of springs and is used to offset a potential setting problem 

that may occur with some shot peened spring designs. The baking is approximately 400 °F (205 °C) for 30 

minutes for carbon steel springs and is below the stress relief temperature of the wire. Temperatures above 

450 °F (230 °C) will begin to relieve the beneficial residual stress from shot peening. 

Other spring designs respond equally well to shot peening. The fatigue failure will occur at the location 

of the highest combination of residual and applied tensile stress. Torsion springs will generally fail at the 

OD near the tangent of the tang. Extension springs will generally fail at the inner radius of the hook. 

Other potential spring designs that can benefit from shot peening are leaf springs, cantilever springs, flat 

springs, etc… 

D R I V E S H A F T S 

Shaft applications are used to transmit 

power from one location to another through 

the use of rotation. This creates a torsional 

load on the rotating member. Since most 

drive shafts are primary load bearing 

members, they make excellent shot peening 

applications. As shown in Figure 5-4 typical 

Figure 5-4 Drive Shaft Schematic 

failure locations for drive shafts are splines, undercuts, radii, & keyways. 

T O R S I O N B A R S 

Torsion bars and anti-sway bars are structural members often used in suspensions and other related 

systems. The bars are used to maintain stability by resisting twisting motion. When used in systems 

subjected to repetitive loads such as vehicle suspensions, shot peening offers advantages of weight 

savings and extended service life. 


AUTOMOTIVE TORSION BARS 

The automotive industry has used hollow torsion bars as a means of weight savings. Shot peening 

was performed on the outer diameter where the highest load stresses occur. On heavy duty 

applications (four wheel drive utility trucks, SUVs, etc…) cracks can also occur on the inner diameter 

(ID) which also experiences torsional loads. 

MIC is able to shot peen the ID using its lance shot peening methods. This provides necessary 

compressive stress throughout the torsion/anti-sway bar length. 

REFERENCES: 


5.2 Lanke, Hornbach, Breuer; Optimization of Spring Performance Through Understanding and Application of Residual Stress; Wisconsin Coil Spring 

Inc., Lambda Research, Inc., Metal Improvement Co. Inc.; 1999 Spring Manufacturer’s Institute Technical Symposium; Chicago, IL May 1999 


C H A P T E R F I V E 

T ORSIONAL F A T IGUE 

25

C H A P T E R S I X 

A X IAL F A T IGUE 

26 

A X I A L F A T I G U E 

Axial fatigue is less common than other (fatigue) failure mechanisms. A smooth test specimen with axial 

loading has uniform stress throughout its cross section. For this reason, fatigue results of smooth, axial 

loaded shot peened specimens often do not show significant improvements in fatigue life. This is unlike 

bending and torsion that have the highest applied stress at the surface. 

In most situations, pure axial loading is rare as it is normally accompanied by bending. Shot peening of 

axial loaded components is useful when there are geometry changes resulting in stress concentrations. 

Undercut grooves, tool marks, cross holes and radii are typical examples of potential failure initiation sites. 


TRAIN EMERGENCY BRAKE PIN 

Figure 6-1 is part of a hydraulic brake 

assembly used in a mass transit system. 

The undercut sections near the chamfered 

end were designed to fail in the event of 

axial overload. During the investigation of 

premature failures it was found that a 

bending load was also occurring. The 

Figure 6-1 Brake Pin Schematic 

combined axial and bending load when simulated in test caused fatigue failure between 150,000 – 

2,600,000 cycles. Shot peening was added to the brake pin and all test specimens exceeded 6,000,000 

cycles without failure [Ref 6.1]. 


AUXILIARY POWER UNIT (APU) EXHAUST DUCTS 

This type of APU is used to provide power to aircraft when they are on the ground with the main 

engines turned off. The tubular exhaust ducts are a high temperature 8009 aluminum alloy welded in 

an end-to-end design. 

Tension-tension fatigue tests measured fatigue strength of 23 ksi (156 MPa) at 3000 cycles in the as 

welded condition. Glass bead peening of the welds resulted in a 13% fatigue strength improvement to 

26 ksi (180 MPa) [Ref 6.2]. 

REFERENCES: 

6.1 RATP, Cetim; Saint Etienne, France, 1996 



F R E T TING F A I L U R E 

Fretting occurs when two highly loaded members have a common 

surface at which rubbing and sliding take place. Relative 

movements of microscopic amplitude result in surface 

discoloration, pitting, and eventual fatigue. Fine abrasive oxides 

develop that further contribute to scoring of the surfaces. Other 

failure mechanisms, such as fretting corrosion and fretting wear, 

commonly accompany fretting failures. 

Shot peening has been used to prevent fretting and eventual 

fretting failures by texturing the surface with a non-directional 

finish. This results in surface hardening (of certain materials) and 

Figure 7-1 Turbine Engine 

Assembly 

a layer of compressive stress. The compressive layer prevents initiation and growth of fretting fatigue 

cracks from scoring marks as a result of fretting. 

Fretting fatigue can occur when a rotating component is press fit onto a shaft. Vibration or shifting loads 

may cause the asperities of the press fit to bond and tear. The exposed surfaces will oxidize producing the 

"rusty powder" appearance of fretted steel. 


TURBOMACHINERY BLADES & BUCKETS 

A very common fretting environment is the dovetail root of 

turbomachinery blades. Shot peening is commonly used 

to prevent fretting failures of these roots. As shown in 

Figure 7-2 the blade roots have the characteristic fir tree 

shape. The tight mating fit coupled with demanding loading 

conditions require that the surfaces be shot peened to 

prevent failure associated with fretting. 

Many turbine and compressor blade roots are shot peened 

as OEM parts and re-shot peened upon overhaul to restore 

fatigue debits otherwise lost to fretting. The discs or wheels 

that support the blades should also be peened. 

P I T T I N G 

Resistance to pitting fatigue is of primary concern for those who design gears and other components 

involved with rolling/sliding contact. Many gears are designed such that contact failure is the limiting 

factor in gear design. Though not desirable, pitting failures generally occur more gradually and with less 

catastrophic consequences than root bending failures. 


Figure 7-2 Turbine Blade and 

Disc Assembly 

C H A P T E R S E V E N 

C ONTA CT F AILURE 

27

C H A P T E R S E V E N 

C ONTA CT F AILURE 

28 

Pitting failures initiate due to Hertzian and sliding contact stresses near the pitch line. When asperities 

from mating surfaces make contact, the loading is a complex combination of Herzian and tensile 

stresses. With continued operation, a 

micro-crack may initiate. Crack growth 

will continue until the asperity 

separates itself leaving a "pit". 

A condition of mixed lubrication is very 

susceptible to pitting failure. This occurs 

when the lubricant film is not quite thick 

enough to separate the surfaces and 

actual contact occurs between the 

asperities. Figure 7-3 shows a gear 

flank and the mechanisms that cause 

pitting [Ref 7.2]. 

Shot peening has been proven to be highly 

beneficial in combating pitting fatigue when 

followed by a surface finish improvement process. By removing the asperities left from shot peening, the 

contact area is distributed over a larger surface area. It is important when finishing the shot peened 

surface to not remove more than 10% of the compressive layer. Please see Chap 11 for photomicrographs of 

a shot peened and isotropically finished surface using the C.A.S.E. SM Figure 7-3 Pitting Failure Schematic 

process. 

G A L L I N G 

Galling is an advanced form of adhesive wear that can occur on materials in sliding contact with no or only 

boundary lubrication. In its early stages it is sometimes referred to as scuffing. The adhesive forces 

involved cause plastic deformation and cold welding of opposing asperities. There is usually detachment of 

metal particles and gross transfer of fragments between surfaces. When severe, seizure may occur. 

Shot peening can be beneficial for surfaces that gall particularly when the materials are capable of work 

hardening. The cold worked surface also contains dimples that act as lubricant reservoirs. The following 

materials have demonstrated positive response to galling with the assistance of shot peening: Inconel 718 

& 750, Monel K-500, and alloys of stainless steel, titanium, and aluminum. 

REFERENCES: 


7.2 Hahlbeck; Milwaukee Gear; Milwaukee, WI / Powertrain Engineers; Pewaukee, WI 


C O R R O S ION F A I L U R E 

Tensile related corrosion failures can be derived from either static or cyclic tensile stresses. In both types of 

failures, environmental influences contribute to failure. Environments such as salt water and sour gas wells 

create metallurgical challenges. In most cases, these environments become more aggressive with 

increasing temperature. 

S T R E S S C ORROSION C R A C K ING 

Stress corrosion cracking (SCC) failure is most often 

associated with static tensile stress. The static 

stress can be from applied stress (such as a bolted 

flange) or residual stress from manufacturing 

processes (such as welding). For SCC to occur three 

factors must be present. 

• Tensile stress 

• Susceptible material 

• Corrosive environment 

Figure 8-1 shows the stress corrosion triangle in which each leg must be present for SCC to occur. 

The compressive layer from shot peening removes the tensile stress leg of the SCC triangle. Without tensile 

stress, SCC failure is significantly retarded or prevented from ever occurring. The following is a partial list of 

alloys that are susceptible to SCC failure. 

• Austenitic stainless steels 

• Certain alloys (& tempers) of series 

2000 & 7000 aluminum 

• Certain nickel alloys 

• Certain high strength steels 

• Certain brasses 

Figure 8-2 depicts a SCC failure. In austenitic 300 

series stainless steel, the “river branching” pattern 

is unique to SCC and is often used in failure 

analysis for identification purposes of this 

material. 

Figure 8-1 Stress Corrosion Cracking Triangle 

Figure 8-2 Stress Corrosion Cracking Failure 

of 300 Series Stainless Steel 


C H A P T E R E I G H T 

C ORROSION FAILURE 

29



30 


FABRICATION OF CHEMICAL HANDLING EQUIPMENT 

Shot peening has been utilized as a cost savings measure for construction of chemical handling 

equipment. In cases where ammonia or chloride based solutions were to be contained, a lower cost 

SCC susceptible material was selected with shot peening rather than a more expensive non-susceptible 

material. Even with the additional shot peening operation, construction costs were less than using the 

more expensive alloy. 

The following table demonstrates the effectiveness of shot peening in combating stress corrosion 

cracking for the following stainless steel alloys. A load stress equivalent to 70% of the materials yield 

strength was applied [Ref 8.2]. 

Material Peened Test Life 

(yes/no) (hours) 

316 SS no 11.3 

316 SS yes 1000 N.F. 

318 SS no 3.3 

318 SS yes 1000 N.F. 

321 SS no 5.0 

321 SS yes 1000 N.F. 

N.F. = No Failure 

C O R R O S ION F A T I G U E 

Corrosion fatigue is failure of components in corrosive environments associated with cyclic loading. Fatigue 

strength can be reduced by 50% or more when susceptible alloys are used in corrosive environments. 


SULFIDE STRESS CRACKING 

Hydrogen sulfide (H 2 S) is a commonly encountered in sour gas wells. Certain metal alloys when exposed to 

H 2 S will experience a significant decrease in fatigue strength. The following test results illustrate the response 

of precipitation hardened 17-4 stainless steel exposed to H 2 S with and without shot peening [Ref 8.3]. 

As Machined and 

% of Yield As Machined Shot Peened 

Strength (hrs. to fail) (hrs. to fail) 

30 29.8 720 N.F. 

40 37.9 561 

50 15.4 538 

60 15.2 219 

N.F. = No Failure 

Testing Done Per NACE TM-01-77 Test Standard 



MEDICAL IMPLANTS 

Medical science continues to evolve in replacing damaged and worn out body components. 

The implant materials (and associated fasteners) must be lightweight and high strength. In 

addition, the human body contains fluids that are corrosive to engineering metals. 

Shot peening has been successfully utilized for combating both metal fatigue and corrosion 

fatigue in alloys of stainless steel and titanium. 

I N T E R G R A N U L A R 

C O R R O S I O N 

During the solution annealing process of 

austenitic stainless steels, chromium carbides 

precipitate to existing grain boundaries. This 

results in a depletion of chromium in the 

regions adjacent to the grain boundaries. 

Corrosion resistance is decreased such that 

the alloy is susceptible to intergranular 

corrosion (sensitized). 

When shot peening is performed prior to the 

sensitization process, the surface grain 

boundaries are broken up. This provides 

many new nucleation sites for chromium 

carbide precipitation. The random 

precipitation of chromium carbides offers no 

continuous path for the corrosion to follow. 

Significant improvements in intergranular 

corrosion resistance have been documented 

with shot peening prior to sensitization. No 

benefit is experienced when shot peening is 

performed after the sensitization process. 

Figure 8-3A is a scanning electron 

microscope image of intergranular corrosion. 

Figure 8-3B shows a primary crack as the 

darkened area and a secondary crack 

propagating through the grain boundaries. 

Figure 8-3a SEM Photo of Intergranular Corrosion 

Figure 8-3b Primary and Secondary Cracking from 

Intergranular Corrosion 

REFERENCES: 

8.1 Figure 8-2, http://corrosion.ksc.nasa.gov/html/stresscor.htm, May 2001 

8.2 Kritzler; Effect of Shot Peening on Stress Corrosion Cracking of Austenitic Stainless Steels, 7th International Conference on Shot Peening; 

Institute of Precision Mechanics; Warsaw, Poland, 1999 

8.3 Gillespie; Controlled Shot Peening Can Help Prevent Stress Corrosion, Third Conference on Shot Peening; Garmisch-Partenkirchen, Germany, 1987 

8.4 Figures 8-3A & 8-3B, http://corrosion.ksc.nasa.gov/html/stresscor.htm, May 2001 




31

C H A P T E R N I N E 

THERMAL F A T IGUE & HEA T EFF ECTS 

32 

E F F E C T S O F H E A T 

Caution should be exercised when parts are 

heated after shot peening. The amount of 

compressive stress that is relieved is a 

function of temperature, time and material. 

Figure 9-1 demonstrates the increasing stress 

relief effect of increasing temperature on shot 

peened Inconel 718 [Ref 9.1]. Inconel 718 is 

commonly used in high temperature jet engine 

applications. 

Stress relief temperature is a physical property 

of the material. Figure 9-2 depicts several 

materials and the temperatures at which 

residual stresses will begin to relax. Many 

shot peened fatigue applications operate 

above these lower temperature limits as 

fatigue benefits are still realized providing the 

operating temperature does not approach the 

stress relief temperature of the material. 

The following are examples where shot peening 

followed by heat treatment is commonly 

incorporated into manufacturing. 

• Springs – It is common to 

perform a post-bake 

operation for improvements 

in spring performance. 

Please see Chapter 5 – 

Torsional Fatigue 

• Plated Parts – It is 

common for shot peening 

prior to plating. Peening 

is called out for fatigue 

benefits and resistance to 

hydrogen embrittlement. 

Please see Chapter 3 – 

Manufacturing Processes. 

Plating commonly involves 

a hydrogen bake operation at 

350-400 °F (175-205 °C) for 

several hours. 


Figure 9-1 Residual Stress Patterns in Shot Peened 

Inconel 718 Alloy after 100-Hour Exposure 

to Elevated Temperatures 

Figure 9-2 Approximate Temperature at Which 

Compressive Stresses Begin to Dissipate

T H E R M A L F A TIGUE 

Thermal fatigue refers to metal failures brought on by uneven heating and cooling during cyclic thermal 

loading. Rapid heating and cooling of metal induces large thermal gradients throughout the cross section, 

resulting in uneven expansion and contraction. Enough stress can be generated to yield the metal when 

one location attempts to expand and is resisted by a thicker, cooler section of the part. 

Thermal fatigue differs from elevated temperature fatigue that is caused by cyclic mechanical stresses at 

high temperatures. Often, both occur simultaneously since many parts experience both temperature 

excursions and cyclic loads. 

REFERENCES: 


FEEDWATER HEATERS 

Large thermal fatigue cracks were discovered in eight high pressure feedwater heaters used for power 

generation. These units operate in both an elevated temperature and thermal fatigue environment. 

Startups and shut downs cause thermal fatigue. Steady state operation is at 480-660 °F (250-350 °C). 

The cracks were circumferential in the weld heat affected zone between the water chamber and tubesheet. 

Fatigue cracking was attributed to years of service and 747 startups and shutdowns of the unit. This 

caused concern about the remaining life of the units. 

The cracked locations were machined and shot peened. Subsequent inspections showed that no additional 

fatigue cracks developed after five years of service and 150 startup and shutdown cycles [Ref 9.2]. 

9.1 Surface Integrity, Tech Report, Manufacturing Engineering; July 1989 

9.2 Gauchet; EDF Feedback on French Feedwater Plants Repaired by Shot Peening and Thermal Stresses Relaxation Follow-Up, 

Welding and Repair Technology for Fossil Power Plants; EPRI, Palo Alto, CA; March 1994 


C H A P T E R N I N E 

THERMAL F A T IGUE & HEA T EFF ECTS 

33

C H A P T E R T E N 

O THER APPL ICA T IONS 

34 

P E E N F O R M I N G 

Peen forming is the preferred method of forming aerodynamic 

contours into aircraft wing skins. It is a dieless forming process 

that is performed at room temperature. The process is ideal for 

forming wing and empennage panel shapes for even the largest 

aircraft. It is best suited for forming curvatures where the radii 

are within the elastic range of the metal. These are large bend 

radii without abrupt changes in contour. 

Residual compressive stress acts to elastically stretch the peened 

side as shown in Figure 10-1. The surface will bend or "arc" 

towards the peened side. The resulting curvature will force the 

lower surface into a compressive state. Typically aircraft 

wingskins have large surface area and thin cross sectional 

thickness. Therefore, significant forces are generated from the 

shot peening residual stress over this large surface area. The thin 

cross section is able to be manipulated into desired contours 

when the peen forming is properly engineered and controlled. 

A properly engineered peen forming procedure will compensate 

for varying curvature requirements, varying wingskin thickness, 

cutouts, reinforcements, and pre-existing distortion. Figure 10- 

2 demonstrates a wingskin that has multiple contours along its 

length. The wingskin is positioned on a checking fixture that 

verifies correct contour. 

Peen forming is most often performed on a feed through, gantry 

type machine (Figure 10-3). 

Figure 10-3 Wingskin Forming Machine 


Figure 10-1 Thin Cross Section Before 

and After Shot Peening 

Figure 10-2 Checking Fixture for Verifying 

Peen Forming of Wingskin 

Peen forming has the following advantages: 

• No forming dies are required. 

• Process is performed at room 

temperature 

• Wingskin design changes are easily 

accomplished by altering the peen 

forming procedure. There is no 

expensive modification of dies 

required. 

• All forming is accomplished using 

residual compressive stress. Peen 

formed parts exhibit increased 

resistance to flexural bending fatigue 

and stress corrosion cracking as a result. 

• Peen formed skins exhibit compressive 

stress on top and bottom surfaces

The majority of aircraft in production with aerodynamically 

formed aluminum alloy wingskins employ the peen forming 

process. 

Metal Improvement Company has developed computer 

modeling techniques that allow feasibility studies of 

particular designs. The program takes three-dimensional 

engineering data and, based on the degree of compound 

curvature, calculates and illustrates the degree of peen 

forming required. It also exports numerical data to define the 

Figure 10-4 Computer Modeling of 

peening that is required to obtain the curvatures. A 

significant advantage of these techniques is that MIC can 

Peen Forming Operation 

assist aircraft wing designers in the early stages of design. These techniques insure that the desired 

aerodynamic curvatures are met with economically beneficial manufacturing processes (Figure 10-4). 

C O N T O UR C O R R E C TION 

Shot peening utilizing peen forming techniques can be used to correct unfavorable geometry conditions. 

This is accomplished by shot peening selective locations of parts to utilize the surface loading from the 

induced compressive stress to restore the components to drawing requirements. Some examples are: 

• Driveshaft/crankshaft straightening 

• Roundness correction of ring shaped geometry 

• Aircraft wing stiffner adjustment 

• Welding distortion correction 

The peen forming process avoids the unfavorable tensile residual stresses produced by other straightening 

methods by inducing beneficial compressive residual stresses. 

W ORK H A R D E N I N G 

A number of materials and alloys have the potential to work harden through cold working. Shot peening 

can produce substantial increases in surface hardness for certain alloys of the following types of materials. 

• Stainless steel 

• Aluminum 

• Manganese stainless 

steels 

• Inconel 

• Stellite 

• Hastelloy 

This can be of particular value to 

parts that cannot be heat treated but 

require wear resistance on the 

surface. The following table 

illustrates examples of increases in 

surface hardness with shot peening. 

Material Before After Percent 

Shot Peen Shot Peen Increase 

Cartridge Brass 50 HRB 175 HRB 250 

304 Stainless 243 HV 423 HV 74 

316L Stainless 283 HV 398 HV 41 

Mn Stainless 23 HRC 55 HRC 139 

Inconel 625 300 HV 500 HV 67 

Stellite 42 HRC 54 HRC 29 

Hastalloy C 18 HRC 40 HRC 122 * 

Hastalloy C 25 HRC 45 HRC 80 ** 

* Wrought condition ** Cast condition 




35



36 

S M 

P E E N T E X 

Controlled shot peening can also be used to deliver a number of 

different, aesthetically pleasing surface finishes. Metal 

Improvement Co. stocks a great variety of media types and sizes. 

These media ranges from fine glass to large steel (and stainless 

steel) balls. Using a carefully controlled process, MIC is able to 

provide architectural finishes that are consistent, repeatable and 

more resistant to mechanical damage through work hardening. 

Shot peening finishes have been used to texture statues, handrails, 

gateway entrances, building facades, decorative ironwork, and 

numerous other applications for visual appeal. When selecting a 

decorative finish, MIC recommends sampling with several finishes 

for visual comparison. Figure 10-5 is a hand rail utilizing a chosen 

Peentexsm finish (left side of Figure 10-5) to dull the glare from the Figure 10-5 Before (right) and 

untextured finish (right side). 

A textured surface is able to hide surface scratches and flaws that 

After (left) Comparison 

of Peentex 

would otherwise be highly visible in a machined or ground surface. It is common to texture molds used for 

making plastics to hide surface defects. The texture on the mold surface will become a mirror image on the 

plastic part’s surface. 

sm 

E N G INEERED S U R F A C E S 

Engineered Surfaces are those that are textured to enhance surface performance. The following are 

potential surface applications achieved through shot peening. 

• In most cases a textured surface has a lower coefficient of (sliding) friction than 

a non-textured surface. This is because the surface contact area is reduced to 

the "peaks" of the shot peening dimples. 

• In some applications the "valleys" of the peening dimples offer lubricant 

retention that are not present in a smooth surface. 

• In some instances a non-directional textured surface is desired over a unidirectional 

machined/ground surface. This has proven effective in certain 

sealing applications 

• In certain mold applications, a textured surface has less vacuum effect resulting 

in desirable "release" properties. 



PNEUMATIC CONVEYOR TUBING 

Pneumatic conveyor tubing can be up to 10 inches in 

diameter and is usually a stainless or aluminum alloy. 

It is used to transport plastic pellets at facilities 

consisting of molding companies or various 

production, blending and distribution sites. 

Transported pellets degrade when contact is made 

with internal piping surfaces. The velocity of the 

pellets results in friction, heat and lost production. 

Using a variation on Peentexsm that produces 

directional dimpling, MIC offers a directionally 

textured surface that significantly reduces the 

formation of fines, fluff and streamers that can 

Figure 10-6 Manufacturing Plant Utilizing 

Directionally Textured Pipe 

account for millions of pounds of lost and/or contaminated production each year. Directional shot peening has 

been found to be much superior to other internal treatments of the tubing, often is more economical and can 

Treatment 

Fines 

(grams/100,000 lb 

conveyed) 

be applied on-site. The directional surface finish has the 

added benefit of work hardening (when stainless or 

aluminum piping is used) extending the life of the 

Directional Shot Peened 

Smooth Mill Finish 

1,629 

4,886 

surface treatment. 

Spiral Groove Pipe 6,518 

The following are test results from six different internal 

Sandblasted Pipe 7,145 

pipe treatments. A lower value of fines per 100,000 lbs 

Polyurethane Coated 

Medium Scored Pipe 

7,215 

13,887 

conveyed is desirable. The directional shot peening 

resulted in one third of the fines of the next closest 

finish [Ref 10.1]. 


FOOD INDUSTRY 

The cheese/dairy industry has found that the uniform dimples provide a surface 

that can advantageously replace other surface treatments. The textured surface 

from shot peening often has a lower coefficient of sliding friction that is necessary 

for cheese release properties on some food contact surfaces. The dimples act as 

lubricant reservoirs for fat or other substances allowing the cheese product to slide 

easier through the mold on the peaks of the shot peening dimples. 

Testing has shown that shot peened finishes meet or exceed necessary 

cleanability requirements in terms of microbial counts. This is due to the 

Figure 10-7 Single Cavity 

Cheese Mold 

rounded dimples that do not allow bacteria to collect and reproduce. Sharp 

impressions left from grit blasting, sand blasting or broken media have proved less cleanable and have a much 

greater tendency for bacteria to collect and reproduce [Ref 10.2]. Both glass beaded and stainless steel media 

have been used successfully in this application. 

Figure 10-7 depicts a single cavity cheese mold. MIC has successfully textured many geometries and 

sizes of cheese molds. 




37



38 

E X FOLIATION C ORROSION 

A large number of commercial aircraft are over 20 years old. Ultimately, the safety of older aircraft depends 

on the quality of the maintenance performed. An aged Boeing 737 explosively depressurized at 24,000 feet 

(7300 m) when 18 feet (6 m) of the fuselage skin ripped away. The cause of the failure was corrosion and 

metal fatigue [Ref 10.3]. 

MIC has developed a process called Search Peeningsm to locate surface and slightly sub-surface corrosion. 

Exfoliation corrosion is a form of intergranular corrosion that occurs along aluminum grain boundaries. It is 

characterized by delamination of thin layers of aluminum with corrosion products between the layers. It is 

commonly found adjacent to fasteners due to galvanic 

action between dissimilar metals. 

The surface bulges outward as shown in Figure 10-8. 

In severe cases, the corrosion is subsurface. 

Once corrosion is present repairmen can manually 

remove it with sanding or other means. Shot peening is 

then applied to compensate for lost fatigue strength as 

a result of material removal. Additional sub-surface 

corrosion will appear as "blisters" exposed from the 

shot peening process. If additional corrosion is found, it 

is then removed and the Search Peeningsm process 

repeated until no more "blistering" occurs. 

MIC is capable of performing the Search Peeningsm on-site at aircraft repair hangers. Critical areas of the 

aircraft are masked off by experienced shot peening technicians before beginning the process. 

P OROSITY S E A L I N G 

Surface porosity has long been a problem that has plagued the casting and powder metal industries. 

Irregularities in the material consistency at the surface may be improved by impacting the surface with shot 

peening media. By increasing the intensity (impact energy), peening can also be used to identify large, 

near-surface voids and delaminations. 

REFERENCES: 

10.1 Paulson; Effective Means for Reducing Formation of Fines and Streamers in Air Conveying Systems, Regional Technical 

Conference of the Society of Plastics Engineering; 1978, Flo-Tronics Division of Allied Industries; Houston, TX 

10.2 Steiner, Maragos, Bradley; Cleanability of Stainless Steel Surfaces With Various Finishes; Dairy, Food, and Environmental Sanitation, April 2000 

10.3 Eckersley; The Aging Aircraft Fleet, IMPACT; Metal Improvement Co. 


Figure 10-8 Exfoliation Corrosion

I N T E R N A L S U R F A C E S A N D B ORES 

When the depth of an internal bore is greater than the diameter of the hole it cannot be effectively shot 

peened by an external method. An internal shot peening lance or internal shot deflector (ISD) method 

must be used under closely controlled conditions (Figure 11-1). Holes as small as 0.096 inch (2.4 mm) 

in jet engine disks have been peened 

on a production basis using the ISD 

method. Potential applications for 

internal shot peening include: 

• Tie wire holes 

• Hydraulic cylinders 

• Helicopter spars 

• Drill pipes 

• Propeller blades 

• Shafts with lubrication 

holes 

• Compressor & turbine 

disc blade slots 

Figure 11-2 Internal Surface Deflector vs External Shot Peening 

Figure 11-1 Lance Peening and Internal Shot Deflector Peening 


C H A P T E R E L E V E N 

MIC developed an intensity 

verification technique for 

small holes. Figure 11-2 

shows the results of a study 

to a jet engine disk comparing 

the residual stress on the 

external surface (peened with 

conventional nozzles) to that 

on internal surfaces of a small 

bore peened with internal 

shot deflector methods. 

Using the same shot size and 

intensity, the two residual 

stress profiles from these 

controlled processes were 

essentially the same [Ref 11.1]. 

D U A L ( I N T E N S I TY) P E E N I N G 

Dual peening (or Dura Peen sm ) is used to further enhance the fatigue performance from a single shot 

peen operation. Fatigue life improvements from shot peening typically exceed 300%, 500%, or more. 

When dual peened, (single) shot peen results can often be doubled, tripled, or even more. 


39



40 

Figure 11-3 Single Peen vs Dual Peen 

Figure 11-3 shows approximately 30 ksi of additional compression at 

the surface when performing dual peening for a chrome silicon spring 

wire [Ref 11.2]. 


The purpose is to improve the 

compressive stress at the outermost 

surface layer. This is where fatigue crack 

initiation begins. By further compressing 

the surface layer, additional fatigue crack 

resistance is imparted to the surface. 

Figure 11-4 SEM Photo of Single 

Peen Surface Finish 

Dual peening is usually performed by shot peening the same surface a 

second time with a smaller media at a reduced intensity. The second Figure 11-5 SEM Photo of Dual 

peening operation is able to hammer down the asperities from the first 

peening resulting in an improved surface finish. The effect of driving 

Peen Surface Finish 

the asperities into the surface results in additional compressive stress at the surface. 

Figures 11-4 and 11-5 show the surface finishes from the single and dual peen at 30x magnification 

recorded in the graph shown in Figure 11-3 [Ref 11.2]. 

T H E C . A . S . E. S M P R O C E S S 

The C.A.S.E. sm process consists of shot peening followed by isotropic finishing. The isotropic finishing 

removes the asperities left from shot peening via vibratory polishing techniques while maintaining the 

integrity of the residual compressive layer. The process is performed in a specially formulated chemical 

solution to reduce processing time making it feasible for high production components. 

C.A.S.E. sm was designed for surfaces that require both excellent fatigue strength and surface finish due to 

contact loading. C.A.S.E. sm has proven quite effective in improving resistance to pitting and micro-pitting 

of gears. Many gear designs are limited by pitting fatigue as the critical factor for load considerations. 

Shot peening is performed to both the tooth flanks and roots with isotropic finishing being concentrated 

on the flanks. Improvements in surface finish allow for contact loading to be distributed over more 

surface area reducing contact stress and extending pitting fatigue life. 

Transmission gears utilized in aerospace, automotive, and off-highway applications are ideal 

applications for the C.A.S.E. sm process. They are expected to run for many years under high root 

bending loads & tooth flank contact loads. C.A.S.E. sm has proven successful for applications in all these

industries. Figure 11-6 shows a typical C.A.S.E. sm finish at a 30x 

magnification [Ref 11.3]. The as shot peened finish would be similar 

to Figure 11-4. The process is designed to leave some of the 

"valleys" from the peened finish for lubricant retention. 

Surface finishes of 10 micro-inches (Ra) or better are attainable on 

carburized gears. Figure 11-7 shows a typical roughness profile of a 

carburized 

gear after 

shot peening 

and also after the isotropic finishing portion of 

C.A.S.E. sm processing. The "peak to valley" of the 

shot peened finish is ~ 2.9 microns. After isotropic 

finishing this improves to ~ 0.6 microns. The Rsk 

following isotropic finishing can approach –1.1 as 

it selectively changes asperities to plateaus leaving 

the valleys from the shot peening. 

Figure 11-7 Surface Roughness After Shot Peening 

and After C.A.S.E. SM Figure 11-6 SEM Photo of C.A.S.E. 

Finishing 

sm 

Surface Finish 

O N - S ITE S H O T P E E N ING 

Large components that have been installed on their foundations or whose size exceed shipping limitations 

can be shot peened by certified crews with portable equipment. Field crews perform shot peening 

worldwide to the same quality standards as MIC’s processing centers. Almen strips, proper coverage, and 

certified peening media are utilized as described in Chapter 12 – Quality Control. 

Examples of portable shot peening projects that have been successfully performed are as follows. 

• Welded fabrications (pressure vessels, crusher bodies, ship hulls, 

chemical storage tanks, bridges) 

• Aircraft overhaul repair and corrosion removal (wing sections, 

landing gear, other dynamically loaded components) 

• Power plant components (heat exchanger tubing, turbine casings, 

rotating components, large fans) 

• Plastic pellet transfer facilities for directional peening. 

• Miscellaneous processing plants (steel mills, paper mills, mining facilities) 

S T R A I N P E E N I N G 

Strain (or stress) peening offers the ability to develop additional residual compressive stress offering more 

fatigue crack resistance. Whereas dual peening offers improvements at the outermost surface layer, strain 

peening develops a greater amount of compressive stress throughout the compressive layer. 

To perform strain peening, a component must be physically loaded in the same direction that it experiences 

in service prior to peening. Extension springs must be stretched, compression springs must be 

compressed, and drive shafts must be pre-torqued. This will offer maximum (residual) compressive stress 

opposing the direction of (applied) tensile stress created during cyclic loading. 

The additional compressive stress is generated by preloading the part within its elastic limit prior to shot 

peening. When the peening media impacts the surface, the surface layer is yielded further in tension 




41



42 

because of the preloading. The additional 

yielding results in additional compressive 

stress when the metal’s surface attempts to 

restore itself. 

Figure 11-8 shows the additional 

compressive stress that is achieved when 

strain peening 50CrV4 [Ref 11.4]. The graph 

demonstrates that more residual 

compression is achieved when more preload 

is applied. While the increased 

compression is desired from strain peening, 

processing costs are higher due to 

fabrication of fixtures and additional 

handling to preload components before 

shot peening. 

P E E N S T R E S S S M – R E S I D U A L S TRESS M O D E L I N G 

When engineering a proper callout for shot peening, MIC considers many factors. One of the most 

important considerations is predicting the residual compressive stress profile after shot peening. The 

following factors influence the resultant residual stress profile. 

• Material, heat treatment, & hardness 

• Part geometry 

• Shot (size, material, hardness, & intensity) 

• Single peen, dual peen, or strain peen 

In addition to MIC’s over fifty years of experience in selecting proper shot peening parameters, internally 

developed Peenstresssm software is utilized to optimize shot peening results. 

Figure 11-9 Typical Peenstress sm 

Residual Stress Profile 

Peenstresssm has an extensive database of materials and heat 

treatment conditions for the user to choose from. Once the 

appropriate material (& heat treat condition) is selected, the 

user then selects shot peening parameters consisting of: 

• Shot size 


Figure 11-8 Residual Stress Induced By Strain Peening 

• Shot material & hardness 

• Shot intensity 

As shown in Figure 11-9, Peenstresssm graphically plots the 

theoretical curve based on the user inputs. By altering shot 

peening parameters the user can optimize the shot peening 

callout to achieve desired results. Peenstresssm contains a 

large database of x-ray diffraction data that can be called up 

to verify the theoretical curves. The program is especially 

useful when shot peening thin cross sections to determine 

anticipated depth of compression to minimize the possibility 

of distortion.

L A S E R S H O T S M P E E N I N G 

Lasershot sm Peening utilizes shock waves to induce residual compressive stress. The primary benefit of 

the process is a very deep compressive layer with minimal cold working. Layer depths of up to 0.040" 

(1.0 mm) on carburized steels and 0.100" (2.54 mm) on aluminum alloys have been achieved. 

Conventional, mechanical peening methods are able to achieve up to 35% of these depths. A secondary 

benefit is that thermal relaxation of laser induced stress is significantly less than that from mechanical 

shot peening of super alloys such as titanium, inconel, etc… [Ref 11.5]. 

Figure 11-10 Lasershot Peening of Al 6061-T6 


Metal Improvement Company entered into 

a cooperative research and development 

agreement (CRADA) with Lawrence 

Livermore National Laboratory when 

developing this process. The process uses 

a Nd:glass, high output, high repetition 

laser in conjunction with a robotic five axis 

manipulator such that a variety of 

components can be processed. 

The benefits of an exceptionally deep 

residual compressive layer are shown in 

Figure 11-10. The S-N curve shows fatigue test results of a 6061-T6 aluminum. The testing consisted of control 

specimens, mechanically shot peened, & laser peened specimens [Ref 11.6]. 

V A L V E R E E D S – M A N U F A CTURING 

Metal Improvement Company manufactures valve reeds for use in compressors, pump applications and 

combustion engines. Valve reeds are precision stampings that operate in demanding environments. 

Tight manufacturing tolerances are required to achieve flatness and offer resistance to flexing fatigue 

and high contact loads. 

MIC employs Stress-Litesm finishing techniques 

that are designed to provide specific surface 

finish and edge rounding requirements for 

durability. For very demanding applications 

Stress-Litesm can be combined with shot 

peening. The following are results comparing 

valve reed performance using Stress-Litesm with and without shot peening [Ref 11.7]. 

• As Stamped: 47,000 cycles 

• Stress-Lite: 62,000 

• Stress-Lite & Shot Peen: 194,000 

Figure 11-11 depicts some of the many 

complex shapes of valve reeds MIC is capable 

of manufacturing. 

Figure 11-11 Valve Reeds 



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R E P R INTS/TECHNICAL A R T ICLES 

Metal Improvement Company has on file a large collection of technical resources pertaining to metal 

fatigue, corrosion, and shot peening. Reprints that are available are listed at the end of this technical 

manual. Please contact your local service center or our website www.metalimprovement.com should you 

desire additional information on shot peening. 

H E A T T R E A T I N G 

Metal Improvement Co. currently operates a network of metal heat treating facilities. These facilities are listed 

on the back cover. In order to best service the varied needs of customers, MIC will make production 

arrangements to accommodate customers who have special heat treating and shot peening requirements. 

REFERENCES: 

11.1 Happ; Shot Peening Bolt Holes in Aircraft Engine Hardware; 2nd International Conference on Shot Peening; Chicago, IL May 1984 

11.2 Lanke, Hornbach, Breuer; Optimization of Spring Performance Through Understanding and Application of Residual Stress; Wisconsin Coil 

Spring Inc., Lambda Research, Inc., Metal Improvement Co. Inc.; 1999 Spring Manufacturer’s Institute Technical Symposium; Chicago, IL May 1999 

11.3 Metallurgical Associates, Inc; Waukesha, WI 1999 

11.4 Muhr; Influence of Compressive Stress on Springs Made of Steel Under Cyclic Loads; Steel and Iron, December 1968 

11.5 Prevey, Hombach, and Mason; Thermal Residual Stress Relaxation and Distortion in Surface Enhanced Gas Turbine 

Engine Components, Proceedings of ASM/TMS Materials Week, September 1997, Indianapolis, IN 

11.6 Thakkar; Tower Automotive fatigue study 1999 

11.7 Ferrelli, Eckersley; Using Shot Peening to Multiply the Life of Compressor Components; 1992 International 

Compressor Engineering Conference, Purdue University 


C O N T R OLLING THE P R OCESS 

Controlled shot peening is different than most manufacturing processes in that there is no nondestructive 

method to confirm that it has been performed to the proper specification. Techniques such as X-Ray 

Diffraction require that a part be sacrificed to generate a full compressive depth profile analysis. To ensure 

peening specifications are being met for production lots, the following process controls must be maintained. 

• Media 

• Intensity 

• Coverage 

• Equipment 

MIC currently meets or exceeds the toughest quality and process standards established by the automotive, 

industrial and aerospace industries. MIC is currently ISO 9002 & QS 9000 certified and/or compliant. 

M E D I A C O N T R O L 

Figure 12-1 

Figure 12-1 Media Shapes 

demonstrates 

acceptable and 

unacceptable 

media shapes. 

Peening media 

must be 

predominantly 

round. When 

media breaks down from usage, the broken media must be 

removed to prevent surface damage upon impact. Figure 12- 

2A (100x magnification) demonstrates the potential for surface 

damage and crack initiation from using broken down media. 

Figure 12-2B (100x magnification) demonstrates what a 

properly peened surface should look like. 

Peening media must be of uniform diameter. The impact 

energy imparted by the media is a function of its mass and 

velocity. Larger media has more mass and impact energy. If a 

mixed size batch of media is used for peening, the larger 

media will drive a deeper residual compressive layer. This 

results in a non-uniform residual compressive layer and will 

correlate into inconsistent fatigue results. Figure 12-3A shows 

a batch of media with proper size and shape characteristics. 

Figure 12-3B shows unacceptable media. 


C H A P T E R T W E L V E 

Figure 12-2a Damaged Surface from 

Broken Shot Media 

Figure 12-2b Typical Surface from 

Proper Media 

Figure 12-3a High Quality Shot 

Peening Media 

Figure 12-3b Poor Quality Shot 

Peening Media 

C ONTROLL ING THE PROCESS 

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46 

To properly remove undersized and oversized media MIC utilizes a 

screening system. To properly remove broken media, MIC meters the 

shot to a spiral separator consisting of inner and outer flights. The 

system is based on the rolling velocity of spherical media versus 

broken media. Shot will arrive via the channel above the cone near 

the top of Figure 12-4. The media will fall to the cone and roll down 

the inner flight. Spherical media will gain enough velocity to escape to 

the outer flight. This media can be reused. Broken down media rolls 

very poorly and will stay on the inner flight where it will be discarded. 

I N TENSITY C O N T R O L 

Shot peening intensity is the measure of the energy of the shot 

Figure 12-4 Spiral Separation 

stream. It is one of the essential means of ensuring process 

System for Shot 

repeatability. The energy of the shot stream is directly related to the 

Media Classification 

compressive stress that is imparted into a part. Intensity can be 

increased by using larger media and/or increasing the velocity of the shot stream. Other variables to 

consider are the impingement angle and peening media. 

Intensity is measured using Almen strips. An Almen strip consists of a strip of SAE1070 spring steel that is 

peened on one side only. The residual compressive stress from the peening will cause the Almen strip to 

bend or arc convexly towards the peened side (Figure 12-5). The Almen strip arc height is a function of the 

energy of the shot stream and is very repeatable. 

There are three Almen strip designations that are used depending on the peening application. More 

aggressive shot peening utilizes thicker Almen strips. 

• “N” Strip: Thickness = 0.031" (0.79 mm) 

• “A” Strip: Thickness = 0.051" (1.29 mm) 

• “C” Strip: Thickness = 0.094" (2.39 mm) 

Figure 12-5 Almen Strip System 


The Almen intensity is the arc 

height (as measured by an 

Almen gage) followed by the 

Almen strip designation. The 

proper designation for a 0.012" 

(0.30 mm) arc height using the 

A strip is 0.012A (0.30A). The 

usable range of an Almen strip 

is 0.004"-0.024" (0.10-0.61 

mm). The next thicker Almen 

strip should be called out if 

intensity is above 0.020" 

(0.51 mm). 

The intensity value achieved 

on an N strip is approximately 1/3 the value of an A strip. The intensity value achieved on a C strip is 

approximately 3 times the value of an A strip (N ~ 1/3A, C ~ 3A).

Almen strips are mounted to Almen blocks and are 

processed on a scrap part (Figure 12-6) or similar fixture. 

Almen blocks should be mounted in locations where 

verification of impact energy is crucial. Actual intensity is 

verified and recorded prior to processing the first part. This 

verifies the peening machine is setup and running according 

to the approved, engineered process. After the production 

lot of parts has been processed intensity verification is 

repeated to insure processing parameters have not 

changed. For long production runs, intensity verifications 

will be performed throughout the processing as required. 

Saturation (Intensity Verification): Initial verification of a 

process development requires the establishment of a 

saturation curve. Saturation is defined as the earliest point 

on the curve where doubling the exposure time produces no 

more than a 10% increase in arc height. The saturation curve 

is developed by shot peening a series of Almen strips in fixed 

machine settings and determining when the doubling occurs. 

Figure 12-7 shows that doubling of the time (2T) from the 

initial exposure time (T) resulted in less than a 10% increase in 

Figure 12-7 Saturation Curve 

Almen arc height. This would mean that the process reaches saturation at time = T. Saturation establishes 

the actual intensity of the shot stream at a given location for a particular machine setup. 

It is important to not confuse saturation with coverage. Coverage is described in the next section and deals 

with the percentage of surface area covered with shot peening dimples. Saturation is used to verify the time 

to establish intensity. Saturation and coverage will not necessarily occur at the same time interval. This is 

because coverage is determined on the actual part surface which can range from relatively soft to extremely 

hard. Saturation is determined using Almen strips that are SAE1070 spring steel hardened to 44-50 HRC. 

C O V E R A G E C O N TROL 

Complete coverage of a shot peened surface is crucial in 

performing high quality shot peening. Coverage is the measure 

of original surface area that has been obliterated by shot 

peening dimples. Coverage should never be less than 100% as 

fatigue and stress corrosion cracks can develop in the nonpeened 

area that is not encased in residual compressive stress. 

The adjacent pictures demonstrate complete and incomplete 

coverage. 

If coverage is specified as greater than 100% (i.e. 150%, 200%) 

this means that the processing time to achieve 100% has been 

increased by that factor. A coverage of 200% time would have 

twice the shot peening exposure time as 100% coverage. 


Figure 12-6 Almen Strip Mounting Fixture 

Figure 12-8a Complete Shot 

Peening Coverage 

Figure 12-8b Incomplete Shot 

Peening Coverage 



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48 

PEENSCAN ® (Coverage Verification): Determination of shot peening coverage can be fairly easy when 

softer materials have been peened as the dimples are quite visible. A 10-power (10x) magnifying glass is 

more than adequate for these conditions. In many applications determination of coverage is more 

difficult. Internal bores, tight radii, extremely hard materials, and large surface areas present additional 

challenges in determining coverage. 

Metal Improvement Company has developed the PEENSCAN ® process using DYESCAN ® fluorescent tracer 

dyes for this reason. PEENSCAN ® is ideal for measuring uniformity and extent of coverage for difficult 

conditions. The whitish-green dye is not visible under normal lighting conditions and must be viewed under 

a UV (black) light. 

The coating can be applied by dipping, 

brushing, or spraying the part under analysis. 

As the coated surface is impacted with peening 

media, the impacts remove the fluorescent, 

elastic coating at a rate proportional to the 

actual coverage rate. When the part is viewed 

again under black light non-uniform coverage is 

visibly evident. The shot peening process 

parameters can then be adjusted until the 

PEENSCAN ® procedure verifies complete 

obliteration of the area of concern. 

Figure 12-9A through 12-9C demonstrate the 

PEENSCAN ® concept. The figures are computer 

simulations of a turbine blade with the green 

representing the whitish-green dye (under black 

light conditions). As the (green) dye is removed 

from peening impacts, the (blue) base material 

is exposed indicating complete coverage. 

Figure 12-9a 

PEENSCAN ® 

Coating Prior 

to Shot 

Peening 

The PEENSCAN ® inspection process has been found to be clearly superior to using a 10-power glass. 

A U TOMATED S H OT P E E N I N G E Q UIPMENT 

Throughout the world, MIC service centers are equipped with similar types of automated shot peening 

equipment. When required, this network allows for efficient, economic, and reliable transfer or duplication 

of shot peen processing from one location to another. 

MIC also offers computer monitored shot peening (CMSP). CMSP is for components that require 

additional documentation above our standard Certificate of Shot Peening as proof of shot peening 

specifications (AMS-S-13165, MIL-S-13165C, AMS 2430, etc…). Parts that are designed with the intent to 

employ the fatigue benefits of shot peening should use the computer controlled processes of AMS 2432. 


Figure 12-9b 

Partial 

Removal of 

PEENSCAN ® 

Indicating 

Incomplete 

Coverage 

Figure 12-9c 

Complete 

Removal of 

PEENSCAN ® 

Indicating 

Complete 

Coverage

MIC has developed CMSP equipment that has the 

capability to monitor, control and document the 

following parameters of the peening process. 

• Air pressure & shot flow 

(energy) at each nozzle 

• Wheel speed & shot flow 

(energy) of each wheel 

• Part rotation &/or translation 

• Nozzle reciprocation 

• Cycle time 

These parameters are continuously monitored and 

compared to acceptable limits programmed into the 

computer. If an unacceptable deviation is found, the 

machine will automatically shut down within one second 

and report the nature and extent of deviation. The 

machine will not restart processing until machine 

parameters have been corrected. 

A printout is available upon completion of the CMSP. 

Any process interruptions are noted on the printout. 

The process is maintained in MIC quality records and 

is available for review. Figure 12-10A is a CMSP 

machine used for peening internal bores of aerospace 

components. Figure 12-10B is a multi-nozzle CMSP 

machine. Both figures show the central processing 

unit on the side of the machine. 


Figure 12-10a Computer Monitored Lance 

Shot Peening Machine for 

Processing Internal Bores 


Figure 12-10b Multi-Nozzle Computer Monitored 

Shot Peening Machine 

CMSP INCREASES TURBINE ENGINE SERVICE LIFE 

CMSP registered significant interest when the FAA allowed a rating increase on a turbine engine from 

700 to 1500 cycles between overhauls. This increase made it possible for the engine designed for 

military use to enter the commercial market. 

There was minimal space for design modifications so the engine manufacturer chose to use shot 

peening to improve life limited turbine discs & cooling plates. CMSP ensured that the peening 

parameters of the critical components were documented and repeated precisely [Ref 12.1]. 



49



50 

S P E C I F Y I N G S H O T 

P E E N I N G 

Figure 12-11 shows a splined shaft (shaded) 

installed with two bearings supporting the shaft 

inside an assembly. The outboard spline and 

adjacent radius would be likely fatigue failure 

locations from bending and/or torsional fatigue. In 

this case, engineering would specify shot peening 

(of the shaft) on the drawing as follows. 

• Area "A": Shot peen 

• Area "B": Overspray allowed 

• Area "C": Masking required 

The details on the print should read 

• Shot peen splined areas and adjacent radius 

using MI-110H shot; 0.006"-0.009" A intensity. 

• Minimum 100% coverage in splined areas 

to be verified by PEENSCAN ® . 

• Overspray acceptable on adjacent larger diameter. 

• Mask both bearing surfaces and center shaft area. 

• Shot peening per AMS-S-13165 

It is important to note that if Non-Destructive Testing is required, NDT should always be performed before 

shot peening. The peening process will obliterate or close up small surface cracks skewing NDT results. 

Metal Improvement Company has over five decades of experience engineering shot peening callouts. 

MIC specializes in the proper selection of shot size and intensity parameters for fatigue and/or corrosion 

resistant applications. The worldwide service center locations are listed on the back cover of this 

technical manual. 

REFERENCES: 



Figure 12-11 Assembly Drawing of Spline Shaft 

Requiring Shot Peening

M I C T E C H N I C A L R E P R I N T S 

1. “Shot Peening of Engine Components”; J. L. Wandell, MIC, Paper Nº 97 ICE-45, ASME 1997. 

2. “The Application of Microstructural Fracture Mechanics to Various Metal Surface States”; K. J. Miller 

and R. Akid, University of Sheffield, UK. 

3. “Development of a Fracture Mechanics Methodology to Assess the Competing Mechanisms of 

Beneficial Residual Stress and Detrimental Plastic Strain Due to Shot Peening”; M. K. Tufft, General 

Electric Company, International Conference on Shot Peening 6, 1996. 

4. “The Significance of Almen Intensity for the Generation of Shot Peening Residual Stresses”; R. Hertzog, 

W. Zinn, B. Scholtes, Braunschweig University and H. Wohlfahrt, University GH Kassel, Germany. 

5. “Computer Monitored Shot Peening: AMEC Writes New AMS Specification”; Impact: Review of Shot 

Peening Technology, Metal Improvement Company, Inc., 1988. 

6. “Three Dimensional Dynamic Finite Element Analysis of Shot-Peening Induced Residual Stresses”; 

S. A. Meguid, G. Shagal and J. C. Stranart, University of Toronto, Canada, and J. Daly, Metal 

Improvement Company, Inc. 

7. “Instrumented Single Particle Impact Tests Using Production Shot: The Role of Velocity, Incidence 

Angle and Shot Size on Impact Response, Induced Plastic Strain and Life Behavior”; M. K. Tufft, GE 

Aircraft Engines, Cincinnati, OH., 1996. 

8. “Predicting of the Residual Stress Field Created by Internal Peening of Nickel-Based Alloy Tubes”; 

N. Hamdane, G. Inglebert and L. Castex, Ecole Nationale Supérieure d’Arts et Métiers, France. 

9. “Three Innovations Advance the Science of Shot Peening”; J. S. Eckersley and T. J. Meister, MIC, 

Technical Paper, AGMA, 1997. 

10. “Tech Report: Surface Integrity”; Manufacturing Engineering, 1989. 

11. “Optimization of Spring Performance Through Understanding and Application of Residual Stresses”; 

D. Hornbach, Lambda Research Inc., E. Lanke, Wisconsin Coil Spring Inc., D. Breuer, Metal 

Improvement Company, Inc. 

12. “Plastically Deformed Depth in Shot Peened Magnesium Alloys”; W. T. Ebihara, U. S. Army, N. F. Fiore 

and M. A. Angelini, University of Notre Dame. 

13. “Improving the Fatigue Crack Resistance of 2024-T351 Aluminium Alloy by Shot Peening”; E. R. del 

Rios, University of Sheffield, and M. Trooll and A. Levers, British Aerospace Airbus, England. 

14. “Fatigue Crack Initiation and Propagation on Shot-Peened Surfaces in a 316 Stainless Steel”; E. R. del 

Rios, A. Walley and M. T. Milan, University of Sheffield, England, and G. Hammersley, Metal 

Improvement Company. 

15. “Characterization of the Defect Depth Profile of Shot Peened Steels by Transmission Electron 

Microscopy”; U. Martin, H. Oettel, Freiberg University of Mining and Technology, and I. Altenberger, B. 

Scholtes and K. Kramer, University Gh Kassel, Germany. 

16. “Essais Turbomeca Relatifs au Grenaillage de l’Alliage Base Titane TA6V”; A. Bertoli, Turbomeca, France. 

17. “Effect of Microstrains and Particle Size on the Fatigue Properties of Steel”; W. P. Evans and J. F. 

Millan, The Shot Peener, Vol. II, Issue 4. 

18. “Overcoming Detrimental Residual Stresses in Titanium by the Shot Peening Process”; T. J. Meister, 

Metal Improvement Company, Inc. 

19. “The Effect of Shot Peening on Calculated Hydrogen Surface Coverage of AISI 4130 Steel”; 

I. Chattoraj, National Metallurgical Laboratory, Jamshedspur, India, and B. E. Wilde, The Ohio State 

University, Columbus, OH. Pergamon Press plc, 1992. 

20. “Effect of Shot Peening on Delayed Fracture of Carburized Steel”; N. Hasegawa, Gifu University, and Y. 

Watanabe, Toyo Seiko Co. Ltd., Japan. 

21. “New Studies May Help an Old Problem. Shot Peening: an Answer to Hydrogen Embrittlement?”; 

J. L. Wandell, Metal Improvement Company, Inc. 


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52 

22. “The Effects of Shot Peening on the Fatigue Behaviour of the Ni-base Single Crystal Superalloy 

CMSX-4”; J. Hou and W. Wei, University of Twente, Netherlands. 

23. “Effect of Shot Peening Parameters on Fatigue Influencing Factors”; A. Niku-Lari, IITT, France. 

24. “Weld Fatigue Life Improvement Techniques” (Book); Ship Structure Committee, Robert C. North, 

Rear Admiral, U. S. Coast Guard, Chairman. 

25. “Controlled Shot Peening as a Pre-Treatment of Electroless Nickel Plating”; G. Nickel, Metal 

Improvement Company, Electroless Nickel ’95. 

26. “Effects of Surface Condition on the Bending Strength of Carburized Gear Teeth”; K. Inoue and M. 

Kato, Tohoku University, Japan, S. Lyu, Chonbuk National University, Republic of Korea, M. Onishi 

and K. Shimoda, Toyota Motor Corporation, Japan, 1994 International Gearing Conference. 

27. “Aircraft Applications for the Controlled Shot Peening Process”; R. Kleppe, MIC, Airframe/Engine 

Maintenance and Repair Conference and Exposition, Canada, 1997. 

28. “Prediction of the Improvement in Fatigue Life of Weld Joints Due to Grinding, TIG Dressing, Weld 

Shape Control and Shot Peening”; P. J. Haagensen, The Norwegian Institute of Technology, A. 

Drigen, T Slind and J. Orjaseter, SINTEF, Norway. 

29. “Increasing Fatigue Strength of Weld Repaired Rotating Elements”; W. Welsch, Metal Improvement 

Company, Inc. 

30. “B 737 Horizontal Stabilizer Modification and Repair”; Alan McGreal, British Airways and Roger 

Thompson, Metal Improvement Company, Inc. 

31. “Residual Stress Characterization of Welds Using X-Ray Diffraction Techniques”; J. A. Pinault and M. 

E. Brauss, Proto Manufacturing Ltd., Canada and J. S. Eckersley, Metal Improvement Company, Inc. 

32. “Towards a Better Fatigue Strength of Welded Structures”; A. Bignonnet, Fatigue Design, 

Mechanical Engineering Publications, London, England. 

33. “ABB Bogie Shot-Peening Demonstration: Determination of Residual Stresses in a Weld With and 

Without Shot-Peening”; P. S. Whitehead, Stresscraft Limited, England. 

34. “The Application of Controlled Shot Peening as a Means of Improving the Fatigue Life of 

Intramedullary Nails Used in the Fixation of Tibia Fractures”; M. D. Schafer, State University of New 

York at Buffalo. 

35. “Improvement in the Fatigue Life of Titanium Alloys”; L. Wagner and J. K. Gregory, Advanced 

Materials and Processes, 1997. 

36. “Effet du Grenaillage sur la Tenue en Fatigue Vibratoire du TA6V”; J. Y. Guedou, S.N.E.C.M.A., France. 

37. “Fretting Fatigue and Shot Peening”; A. Bignonnet et al., International Conference of Fretting 

Fatigue, Sheffield, England, 1993. 

38. “Influence of Shot Peening on the Fatigue of Sintered Steels under Constant and Variable 

Amplitude Loading”; C. M. Sonsino and M. Koch, Darmstadt, Germany. 

39. “Evaluation of the Role of Shot Peening and Aging Treatments on Residual Stresses and Fatigue Life 

of an Aluminum Alloy”; H. Allison, Virginia Polytechnic Institute, VA. 

40. “A Survey of Surface Treatments to Improve Fretting Fatigue Resistance of Ti-6Al-4V”; I. Xue, A. K. 

Koul, W. Wallace and M. Islam, National Research Council, and M. Bibby, Carlton University, Canada, 

1995. 

41. “Gearing Up For Higher Loads”; Impact - Review of Shot Peening Technology, Metal Improvement 


42. “Belleville Disk Springs”; Product News, Power Transmission Design, 1996. 

43. “Improvement in Surface Fatigue Life of Hardened Gears by High-Intensity Shot Peening”; D. P. 

Townsend, Lewis Research Center, NASA, Sixth International Power Transmission Conference, 1992. 



44. “Review of Canadian Aeronautical Fatigue Work 1993-1995”; D. L. Simpson, Institute for Aerospace 

Research, National Research Council of Canada. 

45. “A Review of Australian and New Zealand Investigations on Aeronautical Fatigue During the Period of 

April 1993 to March 1995”; J. M. Grandage and G. S. Jost, editors, Department of Defense, Australia 

and New Zealand. 

46. “Shot Peening to Increase Fatigue Life and Retard Stress Corrosion Cracking”; P. Dixon Jr., Materials 

Week, American Society for Metals, 1993. 

47. “The Effect of Hole Drilling on Fatigue and Residual Stress Properties of Shot-Peened Aluminum 

Panels”; J. Chadhuri, B. S. Donley, V. Gondhalekar, and K. M. Patni, Journal of Materials Engineering 

and Performance, 1994. 

48. “Shot Peening, a Solution to Vibration Fatigue”; J. L. Wandell, MIC, 19th Annual Meeting of The 

Vibration Institute, USA, 1995. 

49. “GE Dovetail Stress Corrosion Cracking Experience and Repair”; C. DeCesare, S. Koenders, D. Lessard 

and J. Nolan, General Electric Company, Schenectady, New York. 

50. “Arresting Corrosion Cracks in Steam Turbine Rotors”; R. Chetwynd, Southern California Edison. 

51. “Rotor Dovetail Shot Peening”; A. Morson, Turbine Technology Department, General Electric Company, 

Schenectady, NY. 

52. “Stress Corrosion Cracking: Prevention and Cure”; Impact - Review of Shot Peening Technology, MIC 

1989. 

53. “The Application for Controlled Shot Peening for the Prevention of Stress Corrosion Cracking (SCC)”; 

J. Daly, Metal Improvement Company, Inc., NACE Above Ground Storage Tank Conference, 1996. 

54. “The Use of Shot Peening to Delay Stress Corrosion Crack Initiation on Austenitic 8Mn8Ni4Cr 

Generator End Ring Steel”; G. Wigmore and L. Miles, Central Electricity Generating Board, Bristol, 

England. 

55. “Steam Generator Shot Peening”; B&W Nuclear Service Company, Presentation for Public Service 

Electric & Gas Company, 1991. 

56. “The Prevention of Stress Corrosion Cracking by the Application of Controlled Shot Peening”; 

P. O’Hara, Metal Improvement Company, Inc. 

57. “Designing Components Made of High Strength Steel to Resist Stress Corrosion Cracking Through the 

Application of Controlled Shot Peening”; M. D. Shafer, Department of Mechanical Engineering, 

Buffalo, NY, The Shot Peener, Volume 9, Issue 6. 

58. “Shot Peening for the Prevention of Stress Corrosion and Fatigue Cracking of Heat Exchangers and 

Feedwater Heaters”; C. P. Diepart, Metal Improvement Company, Inc. 

59. “Shot Peening: a Prevention to Stress Corrosion Cracking – a Case Study”; S. Clare and J. 

Wolstenholme 

60. “Thermal Residual Stress Relaxation and Distortion in Surface Enhanced Gas Turbine Engine 

Components”; P. Prevey, D. Hornbach and P. Mason, Lambda Research, AMS Materials Week, 1997. 

61. “EDF Feedback Shot-Peening on Feedwater Plants Working to 360 ºC – Prediction Correlation and 

Follow-up of Thermal Stresses Relaxation”; J. P. Gauchet, EDF, J. Barrallier, ENSAM, Y. LeGuernic, MIC, 

France. 

62. “EDF Feedback on French Feedwater Plants Repaired by Shot Peening and Thermal Stresses 

Relaxation Follow-up”; J. P. Gauchet and C. Reversat, EDF, Y. LeGuernic, MIC, J. L. LeBrun, LM# URA 

CNRS 1219, L. Castex and L. Barrallier, ENSAM, France. 

63. “Effect of Small Artificial Defects and Shot Peening on the Fatigue Strength of Ti-Al-4V Alloys at 

Elevated Temperatures”; Y. Kato, S. Takafuji and N. Hasegawa, Gifu University, Japan. 


53


54 

64. “Investigation on the Effect of Shot Peening on the Elevated Temperature Fatigue Behavior of Superalloy”; 

C. Yaoming and W. Renzhi, Institute of Aeronautical Materials, Beijing, China. 

65. “Effect of Shot Peening and Post-Peening Heat Treatments on the Microstructure, the Residual Stresses 

and Deuterium Uptake Resistance of Zr-2.5Nb Pressure Tube Material”; K. F. Amouzouvi, et al, AECL 

Research, Canada. 

66. “Transmission Components, Gears – CASE Finishing”; Metal Improvement Company, Inc. 

67. “Steam Generator Peening Techniques”; G. M. Taylor, Nuclear News, 1987. 

68. “Shot Peening Versus Laser Shock Processing”; G. Banas, F.V. Lawrence Jr., University of Illinois, IL. 

69. “Lasers Apply Shock for Strength”; J. Daly, Metal Improvement Company, Inc., The Update, Page 6, 

Ballistic Missile Defense Organization, 1997. 

70. “Surface Modifications by Means of Laser Melting Combined with Shot Peening: a Novel Approach”; 

J. Noordhuis and J. TH. M. De Hosson, Acta Metall Mater Vol. 40, Pergamon Press, UK. 

71. “Laser Shock Processing of Aluminium Alloys. Application to High Cycle Fatigue Behaviour”; P. Peyre, R. 

Fabro, P. Merrien, H. P. Lieurade, Materials Science and Engineering, Elsevier Science S.A. 1996, UK. 

72. “Laser Peening of Metals – Enabling Laser Technology”; C. B. Dane and L. A. Hackel, Lawrence Livermore 

National Laboratory, and J. Daly and J. Harrison, Metal Improvement Company, Inc. 

73. “Laser Induced Shock Waves as Surface Treatment of 7075-T7351 Aluminium Alloy”; P. Peyre, P. Merrien, 

H. P. Lieurade, and R. Fabbro, Surface Engineering, 1995. 

74. “Effect of Laser Surface Hardening on Permeability of Hydrogen Through 1045 Steel”; H. Skrzypek, Surface 

Engineering, 1996. 

75. “Stop Heat from Cracking Brake Drums”; Managers Digest, Construction Equipment, 1995. 

76. “The Ultra-Precision Flappers… Shot Peening of Very Thin Parts”; Valve Division, Impact – a Review of Shot 

Peening Technology, Metal Improvement Company, Inc., 1994. 

77. “The Aging Aircraft Fleet: Shot Peening Plays a Major Role in the Rejuvenation Process”; Impact – Review 

of Shot Peening Technology, Metal Improvement Company, Inc. 

78. “Sterilization and Sanitation of Polished and Shot-Peened Stainless-Steel Surfaces”; D. J. N. Hossack and 

F. J. Hooker, Huntingdon Research Centre, Ltd., England. 

79. “Shot Peening Parameters Selection Assisted by Peenstress Software”; Y. LeGuernic, Metal Improvement 


80. “Process Controls: the Key to Reliability of Shot Peening”; J. Mogul and C. P. Diepart, MIC, Industrial 

Heating, 1995. 

81. “Innovations in Controlled Shot Peening”; J. L. Wandell, Metal Improvement Company, Inc., Springs, 1998. 

82. “Computer Monitored Shot Peening, an Update”; J. R. Harrison, Metal Improvement Company, Inc., 1996 

USAF Aircraft Structural Integrity Conference. 

83. “Shot Peening of Aircraft Components – NAVAIR Instruction 4870.2”; Naval Air System Command, 

Department of the Navy, USA. 

84. “A Review of Computer-Enhanced Shot Peening”; J. Daly and J. Mogul, Metal Improvement Company, Inc., 

The Joint Europe-USA Seminar on Shot Peening, USA, 1992. 

85. “Shot Peening – Process Controls, the Key to Reliability”; J. Mogul and C. P. Diepart, Metal Improvement 


86. “The Implementation of SPC for the Shot Peening Process”; Page 15, Quality Breakthrough, a Publication 

for Boeing Suppliers, 1993. 



87. “Peenstress Software Selects Shot Peening Parameters”; Y. LeGuernic and J. S. Eckersley, Metal 

Improvement Company. 

88. “The Development of New Type Almen Strip for Measurement of Peening Intensity on Hard Shot 

Peening”; Y. Watanabe et al, Gifu University, Japan. 

89. “Interactive Shot Peening Control”; D. Kirk, International Conference on Shot Peening 5, 1993. 

90. “Shot Peening: Techniques and Applications”; (Book) K. J. Marsh, Editor, Engineering Materials 

Advisory Services Ltd., England. 

91. “Evaluation of Welding Residual Stress Levels Through Shot Peening and Heat Treating”; M. Molzen, 

Metal Improvement Co., Inc, D. Hornbach, Lambda Research, Inc. 

92. “Effect of Shot Peening of Stress Corrosion Cracking on Austenitic Stainless Steel”; J. Kritzler, Metal 

Improvement Co., Inc. 

93. “An Analytical Comparison of Atmosphere and Vacuum Carburizing Using Residual Stress and 

Microhardness Measurements”; D. Breuer, Metal Improvement Co., Inc, D. Herring, The Herring 

Group, Inc., G. Lindell, Twin Disc, Inc., B. Matlock, TEC, Inc. 


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shot peening residual stress - Metal Improvement Company

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